They dont cross the tank , now.
My large breeding pair would kill anything I put in their tank but as yours are smaller and the tanks huge, {esp. with lots of rock}, you may be ok.
I once had a pair of orange skunks living in my once featured huge bubble coral. I added a small pair of blue striped cinnamon clowns and they never even looked at each other AND, all four lived in the bubble coral.
That being said, I have seen numerous attempts at adding another clown to a single. Usually ends in the death of the new comer.
Yours are small yet and do have their own home, so it may work, as long as the new pair stays out of their place.