i hve corsair ML 140x2 which can go up to 1200rpm and cooler master 200mmx2 speed up to 700rpm.
mostly people go for higher rpm or play with rpm speed for all the fan setup so the Q is when u said 200 will move a lot of air then what about the speed shortage? and even if i stack 200 and 140 will it disrupt airflow or any disadvantage?