I've recently picked up a 2nd body and am carrying both with me when im out shooting landscapes and wildlife. I have a 5d and XSi so I can have the full frame and crop. I carry a backpack with me but i like to have both cameras readily available. My tripod is either on the backpack or being carried by me.
What i'm looking for is more than likely a double camera strap that will be comfortable to use with a backpack or a single strap that can sort of keep 1 camera out of the way easily but also all for it to come around quickly. The XSi with a 70-200 and 2x tele would be attached to the singe strap.
I've used double strap systems before but if you're going to be hiking, I think you'll probably want something that keeps the cameras from swinging around with each step...the best system I've seen (but have not personally tried) is the Cotton Carrier Dual Harness. It seems this would be a good starting point for you. I've use dual straps systems in the past but I found too much 'camera swinging' for my liking. I now use a Spider holster for a single body and a 2nd camera on a strap...but that my working set-up...I wouldn't go hiking with it but they do have a double holster set-up which you might also want to take a look at. All Spider stuff is VERY well made and (from most reviews that I've read) all Cotton Carrier stuff is equally well made.
I'm thinking the Cotton Carrier Dual Harness is propably more fitting to your needs...of course, it'll make you look like a complete photo-dork
