import AE project with 3d element plug-in effects into another AE project

Have you tried the other way around, i.e. imported the balls into your stick figure project? Also not to pointg out the obvious, but your screenshots look rather crammed, so this may simply be a matter of the computer not having enough juice or meeting the system requirements. What system are you working on and what GPU does it have?

Mylenium

I'm not clear what you mean by "can't edit both at the same time". You need to be more specific and explain what you actually want to do and what isn't working along with the usual niceties like info about your computer, your project in general and so on.

Mylenium