I keep a spare key ON the TW. Having a spare OFF the TW isn't going to help you way too often.
I keep a spare for my streetbike on the bike, a spare for my Banshee on the quad, and a spare for my dirtbike nowhere...'cuz it doesn't need a key.
Oh...and a spare key for my toy-hauler...where? On the rig.
I would encourage you to keep your spare someplace where it will be most useful; on the machine.