Woah! there. A gallon took your pool from 7.6 to 7.4 pH. Two gallons may move it farther than you want. Add a second gallon, let it mix, retest and see where you want to go from there. Maybe all of the third gallon, maybe part of it. I definitely wouldn't take the pH lower than somewhere between 6.8 and 7.0. Then it's time to aerate and wait for the pH to rise.