Two males ,1 on 1, is a tough ratio. There's a lot of potential for domination. I think I'd build another pen if I were you.
As far as the different subspecies...it depends on the individual animals. Easterns are typically larger, but my Three-toes are much more "aggressive". If I ever have a pit fight, I'll let you know who wins. I have a feeling that they'd just stare at me in confusion...or in anticipation of a snack. Most of mine are pretty indifferent towards each other, except when I add a sub-adult to the colony. "Hey, who's the new guy/gal?"
It's best to have at least two females for every male in a colony to act as "buffers". In theory, no one animal will get all of the aggression from bullies.
In your situation, you'd have to either deal with pit-tags or figure out what to do with possible Eastern/Three-toe intergrades if you set up a sexual colony.
If you mix Three-toes and Easterns, it is a good idea to have regular fecal exams, with or without deworming, performed by a Vet. (1-2X per year).
It's a tough call, and it's a judgement on your part.
Here's a tip I learned when breeding Leopard geckos. Now, I use it for my very aggressive NA Wood turtles. If you're creating a 1 on 1 situation, you want to try to provide a neutral location. You don't want one animal to feel as if it's territory is being invaded by an intruder. Overhaul your pen immediately before adding a new individual...move things, replace dirt and leaves, place new structures within. Your Eastern will act as if he's in a new environment, and he may be less protective of his new home. It's a level playing field of sorts.
Let us know what you decide. Good luck.