Back in 2016, Big Finish had a plot line with two different Masters, a year before the BBC brought Michelle Gomez and John Simm together on TV. The first of the three, And You Will Obey Me by Alan Barnes, has Peter Davison's Fifth Doctor involved with Geoffrey Beevers' decaying Master from Traken, at a contemporary English auction of antiques where a grandfather clock, mysteriously escaping the fate of its late owner Mr Masterson, is the object of attention from all, including giant alien dragonflies and a task force of Russians. It's really well done, and Davison is if anything slightly overshadowed by the guest cast.
The second of the trilogy, Vampire of the Mind by Justin Richards, introduces Alex McQueen as a future Master. I knew him as the bad guy's sidekick from Kröd Mändoon and the Flaming Sword of Fire, but apparently he has more recently been an MP in Peaky Blinders (which I haven't seen). He's very good, and so is Kate Kennedy as one-shot companion Heather Threadstone (she was just out of BBC Wales' A Midummer Night's Dream). The setting again in contemporary England; now we are in a disused castle, once a prison and now home to the sinister Dominus Institute (get it?) which of course is central to the new Master's plans. Colin Baker is up to his usual high audio performance standards here as well, and I think I liked this best of the three.
Alas, I felt that the sequence over-reached itself with the closing installment, The Two Masters by John Dorney, where I got thoroughly confused by the plot twists and turns involving body-swapping between the Beevers master, the McQueen Master, and Sylvester McCoy's Doctor, as well as gaps in time and the end of the universe. Lauren Crace, who apparently was in East Enders for a couple of years, is good as another one-off companion, and the repartee between the two Masters (and to a lesser extent the Doctor) very much redeems the piece and made it worth listening to for me. (I should add that fannish reaction in general seems to have been much more positive than mine.) Also if you've listened to the first two you'll want to hear how the stories end.