Big Finish did a "Two Masters" thing, matching Geoffery Beevers' decomposing Master from Tom Baker's era with BF's home-grown incarnation, portrayed by Alex MacQueen. It was okay. I suspect they're doing this mostly so Moffatt can write two Masters sniping at each other as the Doctors do when THEY meet up, without actually doing another multi-Doctor story.
Also, it'd be great if one Master actually KILLED the other because they can't put up with how annoying their preceding/subsequent incarnation is. :P
Mark