I think we are talking about different things here - something isn't adding up. Let me see if I understood You:
Imagine You have one server "A" and two clients - "B" and "C".
1) You create a Wireguard interface on all machines.
2) On the clients You create one peer, and point it to the server. You put the server's public key on both clients peer configuration.
3) On the server You will create two peers: one for machine B and another for machine C. You can use one Wireguard interface for both, or create one interface for each. Your choice.
4) Still on the server, You will put the public key of each client on its respective peer config. The keys must be different.
I never tried to add the same peer on two different interfaces. BUT: Wireguard uses the keys as part of the routing decision. If You have the same public key on two different configs, how would it know where to route to?
Take a look at the reference:
https://www.wireguard.com/#cryptokey-routing