Can a message and one time pad each encrypted with separate ciphers be weaker than the stronger of either cipher?
No, it can cannot be less secure. The proof is simple: if P is ever found out then the security of the strong cipher still protects $P \oplus M$ and thus $M$.

However if the one-time-pad $P$ ever leaks then it can be used to break the weak cipher, and in that case the result is just as secure as the strong cipher. It is generally not a good idea to rely on data to be kept secret forever. And it is certainly not a good idea to rely on security that may or may not be present.

Outside of theoretical security, the weak cipher may also have issues with e.g. padding oracle attacks that could leak $P$, in which case only the strong cipher will be left.

All in all, without additional details, I would rate this about as secure as the strong cipher, so the one time pad doesn't seem to have much benefit (certainly compared to, for instance, frequently using fresh keys).