I had a feeling thats what you were asking

Thats how I use it to vacuum my pool....about 80% from the skimmer and 20% from the main drain. Two things to remember: 1. dont cut off one of the ports quickly, go slow. If you go too quickly it will create a water hammer effect and shake all of the plumbing and possibly crack joints. and 2. there may be a point where your pump is trying to pull more water than what the one pipe (the 80% side, if you will) can support sending to the pump. There are specific numbers (i.e. one 1.5" line flows a max of X GPM), but its best just to watch what happens (the water level in the pump may drop as it cant pull more than one 1.5" line supports). 80/20 split seems to work OK on my system. I lose a little water in the pump basket, but I can actually partially close one of the returns with the other valve and it creates enough backpressure that the pump doesnt try to "overpump" the skimmer line.

Wow that was long winded, hopefully you get the point

-Chris