The motor HP fills the tank. I would connect the two tanks and use the 3 HP motor to fill. You will then have a 24 gal tank with a 3 HP motor. It will take about twice as long to fill but you will end up with more cubic feet of compressed air. You cannot run any type of high cfm (cubic feet per minute) air tool besides a spray gun,nail gun or similar low cfm tool without at least an 80 gal tank and 5 hp motor. If you are trying to run air sanders you need to spend the $s on a bigger compressor.