Two files encrypted with related data versus one file
No, it doesn't lower security, but it doesn't (asimptotically) enhances it either. 
AES (or better, as Thomas pointed out, the way aes is used inside Truecrypt) is secure against IND-CPA attacks, so the encryption of a file itself does not reveal ANY information about the plaintext, except for its length. 

Even if an adversary knows 99 bits of a 100 bits encrypted file (but not the encryption key), it cannot find the last bit without iterating trough the whole set of keys. 
Knowing something about the plaintext does not help the adversary.