Two schools of thought on this one:

1. Just let them get on with it. There may be hissing and growling but it will all sort it self out in the end. Feed them together.

2. Introduce more slowly. Keep separate and scent transfer by swapping their blankets, stroking one then going to stroke other. Let each get a look at the other in a safe environment. You can also make each smell the same by rubbing talc onto each.