If you want to see in the Main comp the effects of changes you make inside a precomp, all you need to do is lock the Comp preview panel while in the Main comp. Then you can double click on your precomp to open it in the Timeline – you will still have the Main comp preview to look at. If you want to preview side by side the Main and the Precomp, you need to double click in the Project Window on your precomp – this will open a Preview in a new tab over the Main Preview. Click on the new tab and drag to the side of the panel until it highlights and release. This will open a new panel for your precomp preview.
Tudor “Ted” Jelescu
Senior VFX Artist