Is it more secure in practice to use the output of multiple hash algorithms instead of a single one (assuming that the output size is the same)?
By secure, in this context, I mean protecting against both collisions and function reversibility.
One particular example would be using 128 bits from SHA-512 plus 128 bits from MD5 instead of using 256 bits from SHA-512.
My intuition is that, theoretically, this would weaken the function, because there are known attacks against MD5 which, effectively, would reduce the output size of the combined function. However, in practice, is it more difficult to find a collision/inversion for both algorithms at the same time - because it'll involve additional theoretical work as opposed to using existing (or upcoming) published ones?
To clarify, I'm simply asking about the basic properties of irreversibility and collision resistance - not about any particular use of hash functions, such as key derivation from passwords (there's excellent coverage about that in other questions) or using the output as unique identifiers.
Another way to phrase this would be "is a hash function defined as the concatenation of two other hash functions better than those?". Also, I'm (somewhat) aware that, information theoretically, it can't possibly be.