Often it works to just throw the pair together - sometimes one of them will start out being aggressive, then a few minutes later the other one has taken over the aggressive role, half an hour later they are coexisting peacefully most of the time and the next day they are acting like a pair.
But the more 'safe' way to do it, is to house them side-by-side for a week or so, preferably separated by a single layer of wire, then let them into an enclosure that is new to both of them, with lots of cover, distractions and places to hide.
The middle road could be throwing them together in an enclosure that's new to both (perhaps just change the layout of the old enclosure).