Combining algorithms, if done properly, isn't necessarily going to make things weaker. But doing it properly isn't trivial (which goes for crypto in general) and the more complex the construction, the more opportunities for screwing it up.

But at the end of the day it's still trying to solve the wrong problem. The chance of a well studied algorithm like AES having a significant weakness is much, MUCH lower than the chance that a particular program using it has a security flaw. Any effort spent layering Twofish and Salsa20 on top of AES would almost certainly be better spent making sure the usage of AES is totally secure.