Before a single screw comes out of the wall, I need to be sure I understand this fully, and rather than type out everything, I made a diagram based on what I was able to dig up on forums. This is for a garbage disposal, the view inside gang is up near the sink, outlet 1 is about 5 feet away and outlet 2 is going to be the newly installed one underneath. Cable 1 is leaving the original box and going down to outlet 2 and cable 2 is leaving the new box and going back up to the original (for the switch) any help and corrections would be appreciated. I hope the diagram is clear enough. Any questions please ask.
Couple things wrong in the diagram - you have 2 neutrals labeled going to the outlet for the disposal and 2 hots tied together. It should look like this:
The garbage disposal is on the right. It's unclear from your diagram, but if there are other GFCI outlets downstream you would connect them to the load terminals on the bottom of your combo and tie the ground in with the rest of them.
One thing I would mention about the garbage disposal though - don't use a normal outlet for it, use a fused outlet like this one:
Ground fault only protects against shock hazards - fused outlets protect against motor stalling (i.e. it gets jammed). Most AC motors will not overload the circuit if they stall, especially a 20A kitchen branch. This will not only protect your motor if you drop silverware or something down the drain, but it is also required by code in many if not most areas. Make sure you size the fuse appropriately for the motor (you'll likely find a fuse rating in the installation instructions for the disposal).