Yes. But there is empty and there is empty. A truly empty tire when rounded out to its full volume is really a vacuum It takes air before it shows a pressure increase as it fill up to 15 lb absolute pressure, 0 lb gauge pressure (15 lb offset by the ambient pressure). That can screw up your additivity manipulations. If you have one truly empty tire and a cartridge fills it to 100 psi, that is really 115 psi absolute. If you divide that among two tires, you will need 15 lb to get to 0 gauge pressure in each. That leaves 115-30 = 85 lb to divide between the two tires or 42.5 on the gauge in each.