There is a similar thread on this topic. I've got a new LG WT1101CW top load washer and our drain pipe can't keep up with the flowrate coming from the washer. It's a great idea to add a larger diameter tube to the drain pipe to allow the excess water time to drain away.
Is there any significance to the second 2" pipe at the outlet side of the trap? Or is this just another drain pipe coming from "upstairs"?