Comment 2 for bug 1097070

I think the easiest way to implement it at the moment would to link it with with the current effects and have a button to turn send/return on and of, then use current FX buttons to add effects to Deck 1 or 2.
The channels to input / output should be configurable in preferences, in the same way as all other ins and outs.

This can latter be built in to a complete effects package where send/return is an effect that can be selected, just like pro mixers and some other software.