everything that dave said is right, but you might be able to get the 23.987 comp to work correctly in a 29.97 comp, if you change a comp setting in the 23.987 comp…
open the comp settings for the 23.987 comp, in the advanced tab select the option to preserver frame rate when nested. this should prevent ae from re-interpolating between keyframes when the 23.976 comp is nested in a 29.97 comp…
again, this may work, or it may not. you may still have an issue if the masks need to actually have an interlaced pulldown to match up with footage that has an interlaced pulldown… but if you just have 23.976 progressive footage that has been masked in the same comp and are bringing that into a new 29.97 comp, this may work, but you will get a duplicate frame every 5th frame…
Kevin Camp
Senior Designer
KCPQ, KMYQ & KRCW