I looked at this several months ago when the UIL announced divisions in basketball. At that time I came up with a cutoff of 65.5 for basketball in order to evenly divide a Div 1 and Div 2. This was 4 students higher than for football due to some of the larger school playing 11-man included in 1A basketball and the fact that the majority of schools not playing football are on average slightly higher than the overall average for 1A.
If they go with the same cutoff for both sports, the UIL may have to fudge the cutoff for football up a point or so in order to keep Div 1 from being 15-20 schools larger than Div 2 in basketball. Bascially cutting the baby in half.