Now, create a multisig descriptor with your and your friend's private key.
The format should be like this if the 2-of-2 MultiSig address starts with "3":
sh(sortedmulti(2,WIF_PrivateKey1,WIF_PrivateKey2))
This assumes OP is willing to enter both private keys on the same system. That kinda defeats the purpose of each having a separate private key.