I travel with my 1Ds MkIII, most of the time with the 100-400, suspended to my neck and the 5D MkII on the shoulder, without any trouble. When I hike and climb I have the 1Ds ready and the 5D in the backpack. The trick is to lock the 100-400 by the waist strap of the back pack, since the problem is not the static weight, but having it jumping around. The big problem with a big and heavy kit as a 1Ds+100-400 is that its inertia is considerable a nd you have to worry also about small impacts against hard object like walls, rocks or poles. Although the body is sturdy, its inertia can be very effective to produce small damages, fortunately usually just aesthetic. Before becoming aware about that, walking in town I let the 1Ds to get in touch with a car rear mirror that produced a small dent in the bottom of the body. No nasty scratches, but it was quite disappointing...