Say we have a message $m$. Signer $S^1$ will sign this message using his digital signature to produce:
Now, say another signer $S^2$ wants to sign the output of the previous stage to generate:
Is there a way to verify that $m$ has been signed by either signer without having to remove the signature of signer $S^2$?
Also, can I verify that $m$ was signed by signer $S^1$, even if I am not interested in verifying that it was signed by signer $S^2$?