In After Effects, one can "pin" the view in the comp window and switch to other comps for editing while seeing the results in the pinned view. In PP, the program monitor wants to always show the sequence that's selected in the timeline. I have looked at using the Program monitor, Source monitor and Reference monitor to achieve the same setup as AE but there doesn't seem to be a way to do this. I'm hoping I'm just missing something because it is slowing down my productivity to be continually switching back and forth between the parent clip and sub-clip to see if the changes I made in the sub-clip are right, especially when you are trying to fit something with pixel perfect location and scale.
Right-click on the 'nest' sequence in the project panel and choose 'Open in source monitor'. Then in the source monitor go to the wrench menu and choose 'Open Sequence in timeline'. Now your program monitor will still be viewing the parent timeline but you can play and manipulate the 'nested' timeline. Not as fast or elegant but it does the job.