I doubt you are going to get any real benefit from crossfire when it comes to using the UE4 editor. All of my experience when it comes to ATI and crossfire is that THEY (ATI) has to add support in on the driver side for you to get ny real benefit. Forcing it in on your driver side will give you a small gain but nothing like when ATI adds in real driver support.
Also I maybe wrong but I think running the editor in fullscreen mode will not trigger crossfire to kick in. It’s not an exclusive fullscreen mode that would kick in crossfire.
Something to check though is that under your project setting > General Settings > at the bottom look and see if Smooth Frame Rate is checked or not.
I use to run two 7870’s in crossfire and it was the worest experience ever playing games. From working sometimes to not working other times. Microstutters, and odd graphic glichs. I was glad when I moved over to two nvidia GTX 780’s.