Time Warner and Comcast on Monday unveiled a plan under which Comcast will begin unwinding a partnership under which Comcast holds a 21 percent stake in Time Warner Cable-setting the stage for both companies to jointly go after bankrupt cable operator Adelphia Communications.

Under the terms of the agreement the two companies reached, Comcast from Dec. 1, 2004, to April 1, 2005, has the right to require Time Warner Cable to redeem a portion of the stake that Comcast holds in the cable unit in exchange for 100 percent of the common stock of a Time Warner Cable subsidiary that will own cable systems serving around 90,000 basic cable subscribers and hold around $750 million in cash.

After the completion of the transaction, Comcast’s stake in Time Warner cable will be reduced to approximately 17 percent from 21 percent.

The move, which has been in the making for months, appears to be setting into motion a plan between the two companies to jointly go after Adelphia, which was officially put up for sale last week.

By tying the unwinding of their partnership with a joint bid for Adelphia, sources said, the two companies could come up with a tax-free way of dissolving Comcast’s stake in Time Warner Cable. At the same time, by teaming up, the two companies could win the Adelphia prize without each having to take a huge hit to their respective bottom lines.