Use both 1.5 drains, but put a valve in one of them and set it just a little lower then the one with out a valve, put a 90 on the tee you have on the valve drain pipe only and point it down under the water, plug the hole in the top so no air can get in, also end them both in the sump under water. open the valve up all the way and start the system running, then start to close the valve a little at a time to slow down the water and raise the level to just under the other drain, issue will be over. If something happens the water will climb and go down the back up never letting the tank over flow.