I don't know what causes split leaves in orchids. In my house plants and out in the garden it's caused by the plant being excessively dry and then suddenly getting drenched. The thirsty plant can't handle the excess water. Don't cut any part of the leaf. Get some adhesive floral tape, (make sure it's adhesive and not the waxed type) which is 1/4" wide and waterproof, and make a butterfly across the split on the underside of the leaf to prevent it from splitting further, and therefore save as much of the leaf as possible. This repair can last as long as a couple of years.