Using two algorithms together is basically just creating a new hash algorithm (with a longer hash), which could also have collisions. If you know a technique f

Using two algorithms together is basically just creating a new hash algorithm (with a longer hash), which could also have collisions. If you know a technique for making SHA-1 collisions and a technique for making MD5 collisions, it's probably possible to combine the techniques and make a collision for SHA-1-MD5.