Is it Possible to Change the Shape of a Video When Creating a Transition?

Not sure the best way to describe what I'm doing, but I'll give it my best shot.

I'm working on a transition right now that has shapes falling from the top to the bottom. While they're falling, I'm cropping video A to reveal video B by the end of the transition. The only problem is, when the shapes are falling, they form a 'V' pattern, and it's noticeable that video A is cropping while the shapes are falling. Is there any way to cut a 'V' shape out of video A so I can hide it behind the shapes while they're falling?

Thanks,

-Tom