Upper half shows one video track, lower half shows another On each of my first two video tracks (V1 and V2), I have a simple PNG image. Both of them have motion effects (position and scale). 

I want the upper half of the output video to show V1 and the lower half to show V2. How do I do this?

I've tried crop but this doesn't work, because the crop "moves along" with the image. I've also tried mask but I run into the same trouble.
The easiest way is to use nested timelines.  Make each source timeline the appropriate size for the half it is on.  Apply the motion within the nested timelines.  Then place the nested timelines on the top and bottom of your final timeline.

I use this technique regularly in both Premiere and After Effects when I need to combine multiple elements into a layer than I then want to manipulate as a whole.  In After Effects, adjustment layers are also an option, but I often need to treat the same sub-asset in different ways in different finished videos (working on lower thirds generally), so the nesting approach makes more sense for me.