What happens when I boil 2 pots of pasta in the same water back to back?
There should be no issue with doing this, restaurants often do this; the only thing that will happen is that the water will have more pasta starch as you cook more pasta in the same pot.  Obviously, basic food safety rules apply here - you need to keep the water out of the "danger zone", but otherwise it should not be a problem.

This could be a good thing as far as helping the pasta bind to the sauce, but will possibly make the pasta a bit more sticky if you don't sauce it right away.