Related comment from jl2012 on bip131 "coalescing transactions" appears at
https://github.com/bitcoin/bips/pull/268#issuecomment-170780308With Schnorr signature it is only possible to combine signature of the same public key, but also different public keys, as long as they are signing the same hash. I think it is the plan after BIP141 is deployed.
Also some other comments from the aethers...
If you have pubkey P1, I can craft P2 = P3-P1, then multisign P1+P2 using P3 all on my own;
P1*H(P1)+P2*H(P2)