The IETF LAMPS draft for X.509 (draft-ietf-lamps-pq-composite-sigs, still an Internet-Draft in 2026) defines combinations such as ML-DSA-44 with Ed25519 or ECDSA P-256 and ML-DSA-87 with ECDSA P-384, each a single OID whose public key and signature concatenate the components with domain separation that prevents stripping one part. OpenPGP's RFC 9980 takes the same approach (ML-DSA-65+Ed25519 is algorithm 30), and OpenSSH 10.4 added an experimental ML-DSA-44+Ed25519 key type.
The goal is a hedge during the transition: security holds if either algorithm holds, and existing classical-only policies can be satisfied at the same time. Alternatives are dual signatures (two independent signatures, as in some CMS and document formats) and pure post-quantum signatures, which NIST IR 8547 expects to be the end state.