Easy, Peasy...
You should really install a 'either, or' switch for the outlet, but using a bit of common sense, you will be fine. The older trailers had one of these switches to change from one load to another, I think the Microwave and A/C, but could be wrong.
I pulled my microwave, installed a 20 amp duplex receptacle for it, made it easier to branch off for the extra outlet but you can just come off the single 20 amp outlet in there and run a 12 gage with ground to the side of the cabinet and install a box and additional outlet. Handy for the waffle Iron, Coffee maker, Hotplate, or?? It is a 20 amp circuit, so it will be nice to have.
Just don't run the microwave while having anything plugged into it.
It probably is not 'up to code' but I like it and have done this with all my trailers and 5th wheels.
http://tinyurl.com/MicrowaveOutlet