Given:
private keys a & b
Public keys A & B
Data to be signed d
Is it possible to create a signature S such that it can be verified given only A, B, and d?
Why not sign the data d with private key a and then sign the result with private key b to give you S.
Use public key B then public key A on S to result in data d.
Would this solve the original problem?
Even though, as pointed out, there are distinct items of data so it wouldn't work in practice anyway.