Not First Introductions?

I had questions concerning sort of, but not really introducing my chinchillas to one another.
Some background information on my boys: Stu is a dark tan male about 4 years old. I've owned him for a little over 3 years. Prior to being with me he was housed with another male and as far as i know never showed any aggression towards his chin friend or his humans. However he came to be with me because his cage mate turned and and bit one of his toes off. thankfully his previous owner was able to separate them until i took him. My other boy, Ein is around 6 years and i've owned him since he was very young. He has never had a cage mate. or had extensive time with another chin. Both are very friendly and social to me.
when i first got Stu he was intended to be a friend to Ein. although i had the means to house him separately if needed. he was quarantined, and placed side by side to Eins cage for 6 months or so until i bought a critter nation. In the six months they were side by side they also had play time together, there was a little mounting here and there and some chasing. Stu seemed more submissive than aggressive. Ein was doing a bit of chasing and mounting. Around the time i got them a critter nation my life got very busy and i was unable to continue introductions. they have been splitting a cn (one on top and one bottom) for 2 or so years now and have had little play time together if not any. What would be the best way to re-start introductions between the two? i want to house them together if possible. although i am completely aware and prepared if i need to keep them permanently separate.