You can generally get away with choking only one carb, as long as the passage between the two is big enough, but you will want to tie the vacuum advance together. Get a 'tee' fitting, and connect the vacuum advance of the carbs together and then tee off to the distributor. My motorcycle has four carbs with the chokes all tied together but it doesn't have balance tubes between the cylinders so it needs it. If you have balance tubes the engine will suck the rich mixture from the choked carb. It just may not be quite as effective as two choked carbs, but it should work in most cases.
