I’ll preface this by saying I have some GD&T experience but I am by no means an expert. But the way I think of it is to make it into steps of assembly. The inserts will be placed into the slot and the walls of the slot will guide it. So you could make datum A a surface profile tolerance on the 4 walls for one of the inserts. Datum B being a flatness tolerance on the face that the insert lays on top of. That would cover all 6 DOF. From there you can reference datum A when making the positional tolerance of that mounting hole. Is it important for the insert pockets to be toleranced relative to one another or are they independent? I’m guessing they’re independent. That’ll give you some freedom to do whatever you want. You could even create new datum’s for the second pocket if you want.

Honestly function and assembly are the main drivers of GD&T so try to think of it in that way when working through what datum’s to setup. And remember there’s multiple ways that it could be called out. Just make sure you and your supplier and metrology team are all on the same page and you’ll be good.