As I understand it, you should use an M2 as the base. This will allow you to use both M2 and M+ as rovers. If you use an M+ as base, then you will only be able to use M+ as rovers and you will not be able to use the M2 as a rover.
This is because the M2 as a rover requires dual frequency corrections from the base (so the base must be an M2, RS2 or other dual frequency receiver)
I second @Africawaterdoc suggestions: if you want to create an RTK network, you’d better go with M2 bases.
You can check our post about receivers’ compatibility for more details. It’s about RS2 and RS+ receivers, but the same rules are applicable for M2 and M+ devices.