Everything I have read online says it not a good idea to tie both drains together for the primary or backup drains. I have tested this as well, with no better results. Some people seem to say it works OK, others not so much. My water level seems to jump all over the place either way it setup. My pump I can control to make it pump only 100 GPH, or 3000 GPH. If I turn it way down, then water spills over the overflow and it really loud and causes a lot of air into the sump. I pulled everything plumbing out last night and started to plumb for durso, not sure what else to do. Friend of my has same setup with 125 and has exact same problem as i do. So not sure what to think at this point.