Yeah you should avoid it really. Not only do you have the risk of them not getting along (and potentially some very nasty injuries) but also one or both may have disease or parasites. This is what quarantine is for, however that will mean there is an extra few months (at least) of them living apart. At the end of the day you may as well just keep them apart.
It's also worth noting that, even though a couple of geks may have lived 'happily' together for years, that is no guarantee that it will stay that way. Which is why it's best to always have a spare setup ready.
Out of interest why are you wanting to house them together?