Why not combine hashing algorithms?
This question is similar to asking whether combining multiple symmetric encryption algorithms is a good idea. Of course it is less likely to get a collision for MD5 and SHA-1 simultaneously compared to just getting a collision for SHA-1. Surprisingly, it is, however, not much more expensive to find a collision to both MD5 and SHA-1 than to find one only for SHA-1, as explained here. Also using two hashes decreases the efficiency because you have to hash everything twice and it becomes more difficult to implement it correctly. Moreover, if you care not only about collision resistance, but also other properties, concatenating two hash functions could even be less secure than using just one of them, as explained here. Overall, it seems to be a better idea to just use one good hash function.