Would it be more secure if we used multiple keys in a One-Time-Pad encryption?
An OTP already provides provable confidentiality in the case that the key is as large as the plaintext, and that the key itself is completely random in an information theoretic sense. Provable security in this case means that even a computationally-unbounded attacker would be absolutely incapable of decrypting the ciphertext if they are not given the key. You can't get more secure than that. Adding another key to an already unbreakable cipher does not make it "more" unbreakable.
Do note that an OTP does not protect the integrity of the plaintext in any way, regardless of how many keys are used on top of each other. An attacker can flip any bit in the ciphertext and know that the corresponding bit in the plaintext will also be flipped. To avoid this, a message authentication code, or MAC, is required. A MAC needs a key of its own, so any extra key material can be used for integrity.