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Activist investors are exerting ever more influence on corporate America, pressing companies to cut costs, replace executives and strike deals. On Monday, William A. Ackman, among the brashest men on Wall Street, made perhaps the boldest move yet for an activist.

By teaming up with Valeant, a big health care company, and offering to buy Allergan, the maker of Botox, for roughly $50 billion, Mr. Ackman has once again found an audacious way to pursue his agenda.

If successful, the joint bid by a hedge fund and a corporate acquirer could provide a new template for how deals are done in an era of increased activity by activist investors.

“This is a harbinger of a much wider range of kinds of deals,” said Ronald J. Gilson, a professor of business law at Stanford Law School. “There are a lot of people with a lot of money who can act very quickly, and they don’t have to do things that look like last week’s deal.”

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But the proposed acquisition, the first of its kind, also raises pressing questions about how activists and corporations work together and how companies defend themselves against hostile bidders. It also opens up potential new conflicts of interest.

Regulatory filings show that over the last two months, PS Fund 1, a part of Mr. Ackman’s Pershing Square Capital Management, has been acquiring shares of Allergan, the maker of Botox and other cosmetic drugs based in Irvine, Calif. During that time, Pershing Square amassed a 9.7 percent stake in Allergan, worth more than $4 billion.

But Mr. Ackman was not acting alone. Valeant, a health care company based in Quebec that is worth $42 billion, had agreed to pursue a joint bid for Allergan alongside Pershing Square. Valeant contributed $76 million to PS Fund 1, filings show, and committed to work with Pershing Square on a joint bid for all of Allergan.

If completed, the deal would unite two pharmaceutical giants during a round of industry consolidation, and provide a major victory for Mr. Ackman.

Allergan shares were up 21 percent in after-hours trading, presenting a potential hurdle to a deal. Valeant shares were up 10 percent, as investors cheered the deal. Allergan declined to comment.

Mr. Ackman had considered teaming up with a hostile bidder for some time, but had yet to find the right partner and the right target, according to a person briefed on the matter.

That opportunity surfaced roughly eight or nine months ago, after Pershing Square hired William F. Doyle, a classmate of Mr. Ackman’s from Harvard Business School. One of Mr. Doyle’s friends was J. Michael Pearson, chief executive of Valeant.

Mr. Pearson and Mr. Ackman met at the beginning of the year to discuss potential partnerships, according to two people briefed on the matter. At the meeting, the Valeant chief confided that he had sought to buy Allergan for more than a year and sought help in trying to buy the company. The two signed an agreement.

Through the arrangement, Valeant gained a highly useful ally: a hedge fund manager with experience battling recalcitrant corporate boards.

Pershing Square then began to buy up shares of Allergan in March and April, quickly accumulating a 4.99 percent stake — just under the legal reporting limit. The hedge fund bought stock in such volume that it pushed up the drug maker’s shares.

Earlier this month, Pershing Square halted its purchases to let Allergan shares settle at what the hedge fund and Valeant believed was the target company’s “unaffected” stock price, roughly $116.63 a share. It then resumed. On Monday, Mr. Ackman’s firm bought the rights to additional shares, bringing its total voting power to about 9.7 percent.

The bid by Valeant is expected to be unveiled on Tuesday morning, a person briefed on the matter said. The company’s board was meeting on Monday night to settle on the exact price.

In the regulatory filing, Pershing Square said that it expected the cash component of an offer to total $15 billion, with financing to be provided by Barclays and the Royal Bank of Canada.

The collaboration between Mr. Ackman and Valeant also acts as a deterrent to a bidding war, with any rival suitor at an immediate disadvantage because of the large stake they have amassed.

Should a deal be completed, Mr. Ackman’s firm would retain a big stake in the combined drug maker, which would have total annual revenues of more than $12 billion.

And unusually, according to the agreement between the two, if a competing bidder emerges with a higher offer that sways Pershing Square, the hedge fund will share 15 percent of its profit with Valeant.

For Mr. Ackman, the collaboration is his latest bold bet. The investment in Allergan is his largest ever — twice the size of his stake in Procter & Gamble at its peak.

Pershing Square is among the best performing hedge funds this year, up 11 percent through the end of March. This deal, if completed, could further lift Mr. Ackman’s returns.

“From Ackman’s perspective, it’s a piece of cake,” Professor Gilson said. “He’s got 10 percent of the stock, and there’s someone who wants the company.”

Yet the deal is atypical for Mr. Ackman, who has built a reputation as an activist investor who buys large stakes in companies he thinks are undervalued, seeking to change the business.

During a three-year-plus campaign to turn around the struggling retailer J. C. Penney, Mr. Ackman made public calls for several senior executives to step down, including the chief executive. The campaign ended with his own retreat after Ronald B. Johnson, the chief executive brought in by Mr. Ackman and the architect of Apple’s retail strategy, failed to revive J. C. Penney’s business. Mr. Ackman lost $473 million on the investment.

In another recent campaign, against Canadian Pacific Railway, Mr. Ackman locked horns with the board of directors, threatening a “nuclear winter” if they refused his demands. When the chief executive finally resigned, five other directors announced they would not stand for re-election, giving Mr. Ackman a major victory. He has since nearly tripled his investment in the Canadian company.

Mr. Ackman has also been engaged in an all-out battle against Herbalife, the nutritional supplements company, having staked more than $1 billion in a bet against the company, which he accuses of being a pyramid scheme.

To help influence the outcome of that bet, Mr. Ackman has lobbied members of Congress and regulators to investigate the company, a move that has helped to push down the value of the stock.

“What’s unusual is that we’re talking about investors who are outsiders — the outsiders are potentially becoming insiders here by teaming up with Valeant,” said Bruce H. Goldfarb, president of Okapi Partners, a proxy solicitor.

For Valeant, already among the most acquisitive health care companies today, a deal for Allergan would be its largest yet.

Mr. Pearson, Valeant’s chief, has set his company an audacious goal of becoming one of the five largest pharmaceutical corporations, as valued by market capitalization, by the end of 2016.

Valeant’s $2.6 billion purchase of Medicis in 2012 bolstered its presence in dermatology and cosmetic treatments, and its $8.7 billion acquisition last year of Bausch & Lomb gave it a big presence in eye care.

But not all its efforts panned out. Last year, it held talks with Actavis about a $13 billion deal that ended without an agreement. And a $5.7 billion hostile bid for Cephalon in 2011 was rejected.

By working with Mr. Ackman, however, Valeant has found a neat way to put the company it wants in play, while risking little.

But the deal could face antitrust scrutiny. Both dermatology and cosmetic treatments are also strongholds of Allergan and the two compete in some markets.

But on the eve of a formal presentation of its case for a deal, Valeant expressed confidence in its plans.

“We firmly believe that combining Valeant and Allergan would create an unrivaled platform for growth and value creation in health care,” the company said in a statement. “And we look forward to finalizing and announcing the terms of our proposal shortly.”

Andrew Pollack contributed reporting.