You'll have to try it. I would definitely try to get one considerable larger than the other if you can, or get them both small as you can safely assume they will both be female then ( they all start female and some of the larger ones become male).
Coral beauties are likely a tiny bit more 'reef safe' than flames, but both are pretty good. If you have sps you'll very likely be fine, and most softies are fine - the corals most likely to suffer predation are some LPS like Trachophyllia, and also clams can sometimes get chewed, but it's not especially likely.