I supposed you could substitute two springs with one, but we always recommend sticking with the same setup you have now to avoid any unforeseen issues. As for calculating sash weight, coil springs are different from other balances in that you want to divide the weight between each spring. For example, if your sash weighs 20 pounds, you want four 5 pound coil springs; if your sash weighs 10 pounds, you want two 5 pound coil springs; and so on.