SeedSigner, check out this project. From their GitHub: "Confirm that cosigners to multisignature bitcoin wallets can sign using signatures. Do they have a key that works?"
nostr:npub1jz0rlhp9ngs3at2kfhzcnc62sxh0y9rxt40x3z003wmdguljky9quaaju6
Discussion
I think it's in Beta now. But it's a really useful tool