you might want to add them at the same time because you have large tang. if you add two at the same time the tang will have more to pay attention to then just one. imo
when you get babies like that there is always a concern because you have to wonder how often do they need to be fed,how are they going to handle the stress of being moved again,how are they going to handle being put in with bigger fish. but i have all aggressive tanks. so if your tang is not aggressive you could probably get away with putting them in sperate. i would ask your lfs
how offten they are feeding the regal tang because some babies have to be fed 3 to 4 times a day because of their metabolism. again this is just my opinion.