MuSig2

A Schnorr-based multisig protocol that aggregates several signatures into a single 64-byte signature indistinguishable from a single-signer transaction. Lower on-chain cost and better privacy than script-based multisig, at the price of an interactive signing round between cosigners. Not used in SSP's default 2-of-2 flow today (which uses P2WSH/script-based multisig), but a candidate for future Taproot-based variants.