Is PSI additive?
 

If I know a certain brand of CO2 cartridge inflates an empty tire to 100 psi, will it inflate 2 tires to 50 psi each?
Or, if I inflate an empty tire with a minipump to 20psi, is it a given that the cartridge would therefore finish the tire to 120psi?

In theory, yes. But most users lose some of the CO2 while inflating.

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.

Assuming a 23-25 road tire, if you are using a small 12g CO2, some air with a pump will get you up to a decent pressure. A 16g should get you there w/o needing the pump. Specific PSI numbers would be hard to judge or repeat consistently. You just need to finish the ride. I would not count on being able to save some CO2 for the next flat, it leaks out once punctured.

Thanks all for the insights. Generally I seat a tube in the tire with just enough air to give it shape.
Does a co2 cartridge once opened leak to extent it can't be counted on during the same ride? Eg. Couple hours or so of elapsed time.

In my experience: yes

either bring multiples or have a mini-pump handy

Meh, you should have a lot left for a couple of hours. But don't depend on it. The cartridges are tiny the way I see it; just carry a few. You can't keep it for another ride for sure.

They are intended for individual use. Carry more than one.