First I would take them both to the vets for a checkup just to make sure they are both fit and well.

Some cats pick on cats which are sick or in pain.

I would also buy Rescue Remedy Disfuser it is a plug in device which is meant to calm cats aggressive behaviour.

I would ring a cat behaviourist and get advice on the situation.

Or you could try a re-introduction....Might help.
i.e. Isolating them from each other in two bedrooms for a week or so, so they start to relax and then slowly introduce them like this http://www.piftails.com/main/article...ingnewcat.html