They were once allies, if not friends.

The entertainment mogul Shari E. Redstone and the CBS chief executive Leslie Moonves, whose careers have been entwined for nearly 20 years, appeared to be getting along famously as recently as February, when they sat in the CBS box at Super Bowl LII in Minneapolis, both rooting for the New England Patriots.

Now, the two media heavyweights are in the early rounds of a bout over the fate of two of the world’s most prominent media and entertainment companies, CBS and Viacom. They find themselves at an impasse because of Ms. Redstone’s desire to merge the two companies, which are corporate siblings. Mr. Moonves, for his part, is firmly against the proposed merger.

At stake is Mr. Moonves’s storied career at CBS, which he took from last place to the most-watched television network, with hits like “The Big Bang Theory,” “Survivor” and “Young Sheldon.”

On Thursday, there were two developments in the dispute. First, a judge ruled against an effort by CBS to block Ms. Redstone from having what is perceived as outsize influence over its board, which had scheduled a meeting to vote on reducing the influence of the Redstone family on CBS. Then, at a meeting later that day, the board voted 11 to 3 to dilute Ms. Redstone’s voting stake to roughly 20 percent from nearly 80 percent.