Superencryption

Hi there,

I'm designing a library using cryptography and I was thinking about using superencryption, more precisely using two different ciphers with two different keys to encrypt a plaintext.
The ciphers would be symetric (stream or block ciphers), well-known (AES, Serpent, etc.) (and guarenteed to produce "pseudo-random-looking" ciphertexts).
I'd like to use two ciphers instead of one not to increase the bits of security, but to protect previously encrypted messages from a sudden break in one of the two ciphers, or a backdoor in the very design of one of them (yes I'm paranoid )
However, I guess using two ciphers can be a very bad idea and actually weaken severely the whole encryption. I'd like to know if someone could give me advices about how to know if two ciphers are "compatible" for superencryption, and how to pick them.

Thanks a lot !