I think most people that play more than one instrument larger than harmonica or tin whistle have to make decisions of what they can carry. I know some multi-instrumentalists on tour make arrangements to borrow instruments at specific venues.

Another difficulty for musicians using public transport is where to leave your gear when not in use. I'm not happy leaving a fiddle (etc) in my tent, but don't like to carry it constantly (esp if dancing!). Festival or campsite organisers should provide secure places to leave valuables.