If it is ONLY two clips you want to work with, it may be easier to just put the second clip after the first on the timeline, and find an appropriate transition or -if you want part of the first clip to remain visible, use the pip designer to move clip 2 in over (part of ) the first clip. If you use borders, you can make it look like collage is being used.
This would even be possible with more than twe clips, but become more time consuming to make it right, but you have more controle to get precisely what you need.