Halving Cake Recipe- Baking Time
What really matters is the thickness of the cake, i.e. the depth of the batter in the pans. If you use two 7 inch pans instead of two 9 inch pans, you have 7^2/9^2 or about 60% the area, and you're putting in 50% as much batter, so you'll end up with the batter about 5/6 as deep. That means the baking time will end up slightly shorter, but in the same ballpark - and just as important, you'll still have layers of a reasonable thickness! The recipe says 23-27 minutes, so you might start checking around 18-20 minutes to be safe.

(If it were giant you might to worry about the diameter/width of the cake too, but not for something like this.)