The only way, and I done this multiple times is if you do a b2b cruise, the
DCL agent can apply one placeholder to first let and the 2nd room to the second leg. (I.e. both place holder was from the same original reservation) Yes, the rule is up to 2 place holder per household that must be used on the same sailing. With the loophole being b2b.
One lead person name listed in the placeholder must be on that sailing and in that exact room. Once you get onboard, you can reshuffle if you want.