No need for multivendor with a DIY seedsigner as it's stateless, there's almost no attack vector. You can create a multisig wallet with one seedsigner.
https://econoalchemist.github.io/SeedSigner/07_Multisig.html
Discussion
Thats true; I'm also a seedsigner maximalist. Call me paranoid, but when working with multisigs I like to have another stateless signer - running on a completely different software and hardware stack - to verify everything is working at it should.
My coldcard mk3’s only use case is being my seedsigners bitch