They don't combine the highest sections from separate MCAT scores.
So example: They don't do below which is what I think you are asking
1st test(128,128,120,128) Total :504
2nd test(124,124,129,123) total: 500
Mcat for user(128,128,129,128): 513
Instead what some schools is pick the highest MCAT but I believe most schools average out the two or however many tests you take.
So for many schools (504 + 500)/2 = 502 or other schools just take 504