I admit to having done this (using two plugs to get 220V from 110V outlets) but I wouldn't set such a thing loose on the public. I connected both green wires together to get the ground for the 220V socket, and one black wire to one of the hot contacts of the 220V socket, and the other black wire to the other. *HOWEVER* there is a serious safety hazard in that if one of the plugs to the wall plugs is pulled, and the other not, and if the load is still turned on, you have a male plug with a hot prong to shock you. So I do not recommend that anyone ever do this. I did not connect the neutrals together or to anything else.
