That would be a little over 6 amps (75w/12v = 6.25a) draw, which would likely create a heat issue in the wiring. Most wiring for those 12v outlets is not capable of pulling a lot of amps, combine that with the problem of small gauge wires to the outlet, and then consider that the length of the wire covers a good distance and you'll find that the resistance is just too high to not melt/burn when you are pulling a lot of load. Plus, it's not recommended to load any given circuit to it's full rating (%80 is the actual max, for safety) so you're getting way out of spec with both appliances running.
It would be better to run new, heavier gauge wiring to each device. Use a wiring calculator (
http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html) to determine the gauge wire recommended for that load and that distance (length of wiring from the battery to the outlet) so that you know you won't have issue.