If buying new equipment is at all an option i would recommend just carrying a 24-70mm and a 100-400mm lense. That would be a lot less weight and not reducing your range by much. If not how are you carrying your equipment? Proper weight distribution can help heavy not seem as heavy. I throw my extra lense on a hip-belt holder, my camera(Sony A7R) and 24-70mm lense is on my backpack strap with a Peak Design camera clip then I'd throw your 500mm in the bottom of a backpack. Extra batteries, shutter remote, cleaning equip go in my pockets or backpack hip strap pouches(or a fanny pack). Tripod in backpack too on a side holder for easy access but that only weights 1.5lbs haha.