The traditional method of "manifold splitting" doesn't actually cut it into two pieces. A slot is sawed or ground into the manifold runner just ahead of the heat riser area, and a piece of steel or cast iron is cut to fit the slot and welded inside and out to block off most of the manifold runner. Some people choose to make the split slightly leaky to allow a little bit of exhaust gas transfer from front to rear; others block it off entirely. Then a second flange is added to the front section for another pipe. You'll need to observe all the cast iron welding precautions- - - -proper alloy welding rod, pre and post heating, peening the weld beads for stress relief, and then bury the manifold in a pile of sand to cool down slowly and prevent cracks. Fun and games! I just recently found some nickel-allow wire for my MIG welder but I haven't tried welding any cast iron with it yet.
Jerry
