I keep separate bags, but the spare batteries, or exposure meters or ... always seem to be in one of the other bags, e.g.

2xContaxII 28mm, 35mm, 50mm, 85mm, 135mm, 2x viewfinders, yellow, orange, pola filters, concentric cassettes.
3xCanonP similar
3xLeica M2 similar

Sometimes carry spare film is knapsack with wet weather gear and monopod...

Noel
P.S. Way too much weight for comfort but it is more interesting than going to gym.