He has taken out loans to buy up shares in Tesla and SolarCity, some backed by his personal stock holdings in both companies — a risky move that leaves him exposed to margin calls if their stock prices slide too far. He has defended the practice as low-risk to other shareholders, given the sheer size of his personal net worth of more than $10 billion.

In Mr. Musk’s view, putting Tesla and SolarCity together is only logical.

“We need to achieve a tight integration of the products,” he told reporters in a conference call on Tuesday. “I think it’s an obvious thing to do.”

An agreement is some time away, if one is ever reached. But shareholders in SolarCity pushed the company’s stock up 19 percent in after-hours trading, to $25.26. Shares of Tesla, however, tumbled more than 13 percent, to $190.59.

Both companies have been growing fast, but have also consumed enormous amounts of cash to pursue their goals.

Of the two, SolarCity — where Mr. Musk is chairman and his cousin, the co-founder Lyndon Rive, is chief executive — has been the more troubled, buffeted by changes in the regulations on the solar energy industry. While an important federal tax credit was extended, local policies have cut into the savings that solar providers have promised.