Lock playhead in time between multiple comps:

[avclubvids]

I often have keyframes in a precomp that need to be adjusted while looking at the result of those keyframes in another comp (this is more often than not in AE). We can step frame by frame or jump around and see what we need to see, but we cannot scrub or play and have both comps play back or scrub, so there is a lot of back and forth. I'd love it if there were an option to lock playheads and preview/scrub multiple comps simultaneously to speed this workflow up.

[Rick Gerard]

Try selecting the Composition panel and then use the View Menu or the keyboard shortcut Shift + Alt/Option + Ctrl/Cmnd + N to Split With New Locked Layer. Check to ensure that Syncronize Time of Related Events is turned on on the first page of Preferences. Easily open Preferences by pressing Alt/Option + Ctrl/Cmnd + ;

Only one Comp viewer will preview at a time, but when you are playing the Main comp (the locked viewer) you can go to any of the nested comps (pre-comps) and make adjustments and the preview will update. 

Here's a sample from one of my recent posts:

That will setup will allow you to look at one comp while adjusting the other.

[avclubvids]

This is how I do it now, but not what I am talking about. I want to scrub/play two preview windows at the same time, or at bare minimum scrub/play the timeline of one comp and have the viewer for a different comp sync to it. Right now, I can only get what I want by stepping through one frame at a time; as soon as you hit play, it pops over to the main comp (as defined by the primary viewer option and/or the lock).