Sounds interesting to me but would be difficult to pull off...
Remember the Imperium arn't known for being very tolerable so if a squad of marines appeared asif from nowhere and starting claiming Horus killed The Emperor they would probably be shot on sight
All depends on if the Imperium are aware of parallell universes, im not sure on that one tbh.
But all in all a good idea, and definatlely would make for interesting models
the purist marines would have to be in cracked, dirty hand me down power armour seeing as they can't really replace it, wheras the chaos marines would be glistening.
Let us know when more comes up