The only safe way to get a pair is to buy 5 or 6 of them and let a pair form naturally, then return the spares back to the shop (or sell them) before the pair kill them all. The other way to do it which is less successful is to keep a confirmed sexed male/female with a clear divider seperating them, this allows them to get used to each other without the aggression normally seen. But this doesnt always work, i lost a male gachua the first time i tried this method as the female didnt take to him once the divider was removed.