If you have a one-handed item in both the L and R slots, they'll both be out at the same time. If you have a one-handed sword in R and you switch your left hand to another slot with a one-handed item, say the flashlight, you'll still have the sword in your right hand.
X will immediately switch you back to the L/R slots and Z will switch the item between the hands. I generally keep my primary weapon(s) in these slots.
Food is generally the superior healing option once you're farther in the game. Proper recipes, anyway. Normal bread or some simple meat won't do much for you(that is to say, it'll do nothing for you in terms of health). Carrot bread, pancakes and so on are great, however. Stim packs are also an option but they're only available from chests, some NPC's and merchants - they also become less desirable in the really high tiers, though.