Emulate iMovie PIP exchange effect in Final Cut Pro X
You have to create the effect in Motion.  I'm not sure what the iMovie effect looks like, but I use this technique on a daily basis. In my setup, there are actually six separate effects; left_panel_in, left_panel_static_, left_panel_out, right_panel_in, right_panel_static, right_panel_out.  This allows me to go between a 1-up and 2-up configuration at any point in time, and to do so consistently throughout multiple projects.  If you need step by step instructions, I would be happy to assist once I'm sitting in front of a machine with Motion on it.

best, Jason.