There are 2 new elementary schools opening in the next school year in our area. The school district just went thru a major reassignment and some of our students are being reassigned to these 2 new schools.
The PTA president has asked if we want to give money (or something else) to these 2 new pta's.
What is customary?

I'd say it depends on a few factors:

1) If this year's fundraising money goes towards next year's expenses (your PTA works a year in advance to fund the budget), then the parents who gave this year did so with the expectation that the money would benefit their children. If that's the case, seed money for the new PTAs would be highly appropriate.

2) If, however, this year's activities funded this year's expenses, your PTA will theoretically will start with an empty pot next year just like the new PTA's will. In this situation, "every man for himself" would not be an unjustified position to take.

Those factors aside, if your PTA can afford the seed money without putting teacher/school needs in jeopardy, providing it would be a very generous gesture of goodwill. Perhaps 1-2% of your budget would be an appropriate figure.