I was on safari in south africa 2 years ago, with my wife who also shot.
we had with us:
- 5d mkii
- 300 2.8is (+ 2xiii)
- 70-200 4 is
- ex 580
- strong tripod with gimbal
- enogh cards and batteries
All of this fits in one backpack which goes as cabin baggage, its weigts about 15kg
Most of the time we used 50d with 300 w/wo 2x and 5d with 70-200 or 24-105. The flash was used for some snapshots of humens (i didn't flash animals) and 17mm for some landscapes and in cities. In low light the 300mm was used on the 5d
keep things as simple as possible and dont shoot to many dublicates, it makes less sense to have two similar lenses on two different bodies, better complement each other. With this said, the 300mm is worth gold, and the wider perspective is done with the other body. If one body breaks only one person can shoot, but the other one can spot and help change lenses.
Dust was no problem, we just had a professional cleaning set, but one must practice to use it in advance.
Practice with 600mm on crop, its not easy to find and track the "target" if it moves, just try to shoot some common birds for practice.
In doubt dont use the extender, in critical light its better to crop the final image than to shoot at high iso and add motion blur and camera shake.
Dont forget to enjoy your trip with your eyes not just trough the viewfinder.
With some training i was able to carry the backpack with me the whole day, but it was the limit. i wouldnt have liked to carry a bigger tele, or additional lenses.
Viewing back, i would have left the flash at home and taken a fast 50mm with me instead, we missed some impressive night action because i didnt have it.