If the keys used for each cipher are independent it should not weaken security. See section 15.7 of Applied Cryptography (http://www.cse.iitk.ac.in/users/anuag/

If the keys used for each cipher are independent it should not weaken security. See section 15.7 of Applied Cryptography (http://www.cse.iitk.ac.in/users/anuag/crypto.pdf http://www.cse.iitk.ac.in/users/anuag/crypto.pdf).