I've kept them together for years. If they are the only 2 zebrasoma tangs you may have issues - This would be the same with any 2 zebrasoma tangs though. In the zebrasomma family the Gems and Purples are definitely the most aggressive in my experience (3.5 decades). Purples have a tendency to be long term established fish and then after years of chilling get a sudden growth spurt and as they all of a sudden start outgrowing the other tangs - Go all Gangsta on the smaller tangs.
Gems on the other hand are usually a little more 'fiesty' in general. While they are not as agressive in their attacks as purples, they will be alot more active at close range nips at other tangs.
First off - If you don't have any other zebrasomma proceed with extreme caution - if you have the ability to quickly remove the least favorite fish and house it safely till it recuperates.
Get a few other zebrasomas, scopas or sailfins, yellows (if Biota sizes are comparable) and give them more than just each other to chase after. The more fish you have, the more chaos, the less aggression.
When you get the Gem, get it slightly (just slightly) smaller than the purple - because usually they will be the aggressor.
If you are just getting started with your tank, as you posted, then stock the tank with everyone else and get the most amount of distraction and chaos before adding the Gem. The distraction of other fish - so they don't focus on just each other - is going to be super important.
Dave B
