It could be nice & friendly in that tank, but things could change quickly in another tank.
The fish in the new tank will have to develop a ranking system, & there could be some deaths.
If your 2 Midas/Midevils work together I'd just work with them.
Since you don't know the sex of your 2, I wouldn't risk trying to add another fish.
I'm not saying it won't work, but it's a big risk if your 2 end up being females, & the new one is a male.
It would be pretty nice if it did work for you though.
If you do get this fish, make sure you have a backup plan if thing turn ugly.
I'd hate to see your fish injured, or worse.
(Since I luv your thick fish)