I suppose it is possible, but it might be more trouble than it is worth (you would essentially have to purchase another CO2 regulator and paintball tank in addition to some hardware to connect the two together).
It would be easier to just buy a spare cylinder and switch it out as appropriate.
Yes, you can use more than one CO2 tank hooked to one regulator. The disadvantage is that you would have no way to know if one of the CO2 tanks was empty, so effectively, you would have to refill all of the tanks at the same time. And, of course, it takes high pressure plumbing to connect all of the tanks to a regulator - think Swagelok fittings, for example.
It's definitely possible. Just have to find the right fittings. We connect high pressure tanks all the time in scuba. You'd need a cutoff valve for each so all the fittings can be connected before being pressurized. You'd end up with essentially one single tank with twice the capacity. It's only empty when both are empty unless you have a serious manifold setup that can individually control each tank's pressurization.
I agree with PlantedRich that's really not worth it, though. A LOT of hassle and expensive fittings for only 2x paintball capacity? Way better, and probably cheaper, to get a real co2 tank (5 lb or more) that hold way more than a couple of paintball canisters can.