Here's the thing - where I am, we don't use the AWG wire sizing, so I'm not that familiar with it - I found a chart that shows ampacity, and 16ga is rated at 22A, so I'd say you're safe - just parallel the wires and crimp them.
Now, the bumper will typically bolt directly to the chassis, so grounding would not be a problem there, but I'm not certain how your roof rack attaches to the vehicle - so you might want to run a ground wire between the rack and the chassis.