Travelling with two cameras opens a plethora opportunities for photographer, besides rather obvious backup benefit. Switching cameras is faster than switching lenses. Normally I travel with full frame Canon 6D and crop Canon 760D as well as a few mostly prime lenses. Using full frame lens on a crop body is a great force multiplier since for one lens you get two focus distance possibilities.
My favorite travel combinations (I favor wide angle):
Canon 760d: ef-s 10-18mm, ef-s 24mm f/2.8.
Canon 6d: ef 24mm f/2.8, ef 28mm f/1.8, ef 35mm f/2.0, ef 40mm f/2.8, ef 50mm f/1.8