Your model is too simple. Let's prove, that Satoshi participated in your message:
P3=03678AFDB0FE5548271967F1A67130B7105CD6A828E03909A67962E0EA1F61DEB6
R3=02DE3DCE2CDA1208E97C20BB537BB13E349A853463AD4DBA1FA20501979B87D495
s3=0000000000000000000000000000000000000000000000000000000000000001
P4=0249A5B691630895861D099B168950F9E8A38289D90657EFFF6C30B121F4C5E208
R4=03BADE9D75A6D917A5D8BA2A26D1F1AED70E59E50E0C87D366CFAD5BC4D9B8D730
s4=9308a87057ca70d6b6c597f5043686f2e2fa8487e05e6e46e510528b404da728
P=P3+P4
P=0256B76A89AF551C6C58B1972EE5BE3DAE2A2A39202AD7439962F647780F025E6A
R=R3+R4
R=025DA08419189F4C48912F64130DC262C522EBE81E10663927FB1341EC41E208E8
s=s3+s4
s=9308a87057ca70d6b6c597f5043686f2e2fa8487e05e6e46e510528b404da729
Joined parts should not be used directly as a proof, by checking just your public key. You should instead hash all public keys, and put it into R-value, and then prove it, by revealing a commitment. In other case, you can prove, that anyone was a part of your multisig.