The protocol doesn't support multiple signatures. That would create an invalid event.
Here is a howto for the musig2 scheme 
GitHub
nak/README.md at a6509909d0d83405a5be57fbc1831f6bb96b316e · fiatjaf/nak
a command line tool for doing all things nostr. Contribute to fiatjaf/nak development by creating an account on GitHub.