I can't find the paper where it was discussed, but I remember reading through where someone tested four different methods for introducing/pairing broodstock. 3 of the 4 were pretty effective for getting them to pair while the fourth had frequent aggression.
I only remember two methods off-hand: one method was just dropping them into the same tank; the other was putting them in breeder boxes (or breeder box like setups) where they could see each other and sense the chemicals in the water from each other but not actually interact with each other physically. Dropping them in is the one that often resulted in aggression, so I wouldn't recommend that. I'd introduce them through the breeder box method. In theory, you should be able to tell if they're aggressive/calm and if they're bonded or not within a few days/weeks. (That said, these methods may be more effective on fish that do more swimming and less sitting in a burrow; I'm not sure, but I'd be curious to find out.)
