If you use a standard cable, the send will automatically sum. Standard cable from send to return (mono) will give a summed signal. I frequently use external overdrive pedals like the OCD in the chain before the amp model and have never had issues with mono/stereo when doing that. When I want to use full stereo effects in the loop, I just use a small Y cable that takes a stereo plug and splits it to separate left and right plugs. Very simple.
