There is no rule that says an actor cannot be part of both the Marvel Cinematic Universe and the DCEU, something history has proven countless times.
Ryan Reynolds, Michael Keaton, Chris Evans, Christian Bale, Josh Brolin, Taika Waititi, Hugo Weaving, and Idris Elba are just a few of the names that have tackled the two Hollywood comics monoliths at one stage or another, but it would be nonetheless be a twist for the books if a newcomer to one franchise simultaneously got a key part in the other.