I believe the biggest consideration of not having it on the screen wall is localization if crossing higher than 80hz. Some find localization at 80hz, especially if running the subs hot. It may even measure best on the rear wall if you get the eq right, you will need measuring equipment to test this.
I would be curious to hear others comments on if this would be much less of a concern since you are running duals in opposite corners, filling the room more??
The big plus I can see would be the nearfield placement for a more visceral impact response.
I have considered this configuration myself if I plan to add a second sub at some point. I currently have an svs pc13 behind my front speakers up front. It measures best there, but looks out of place as I have to push my speakers out to fit it behind. I have considered the dual rears strongly over the past couple of years, but have yet to pull the trigger to try it.