Its taken about 2yrs for my emp. to grow from 3" juv. to a 7-8" adult with 90% full colours. A queen would likely grow a little bit quicker.
So - if you put two juv. fish, at about 3", into your tank I reckon you've got a good 2yrs before they are adult(ish) and at that size (7-8"+) at which point I would think you would need to start planning an upgrade or else selling on one of them. At that time, you may find they co-exist peacefully - then again they may not...... but you should have 2yrs grace anyway, at which point the fish will decide for themselves if they want to live together or not.
HTH
MAtt