Your long Zoom makes it easier to work with one camera, but I'd still want two bodies.
The 120-300mm on one, and something in the wider end of things on the other.
Being able to go from this;
IMAGE LINK: http://jakehegnauer.zenfolio.com/p326415976/e54cb074a
to this,
IMAGE LINK: http://jakehegnauer.zenfolio.com/p326415976/e577f85fa
......without fumbling to swap lenses is a fundamental requirement.
I notice you don't have that sort of lens either, with 50mm prime being your widest, so Jim_T's suggestion is even more on target for you.
Otherwise, a 2nd body and a zoom like the 24-105 or even 17-40mm might be worth considering. Or one of the excellent affordable EF-S zoom options.
I for one rarely try to shoot with just one body in situations where I need to react fast. For Africa I always had two bodies at the ready. Ideally, if I had the space and cooperation of those around me, I'd actually have three.
A long prime, a long zoom, and a wide /mid zoom. But three is very hard to manage.