Ziplocks are a bad idea for digital -- in fact plastic of any kind is a bad idea for digital because of static and outgassing. Best is probably form-fitted and sealed anodized aluminum or stainless steel.
The film back is less critical since you get a "fresh" sensor with each frame. I always relied on the darkslide for protection and just blew it off before mounting.
Personally, If I were shooting film and digital together in the field, I'd get a second body and dedicate one to each. Then you have back-up if one goes down...
Just to clarify, I would not be storing the either back in a ziplok. The digital back came with a nice fitted soft case. So I would use that for both.
I was not to too concerned with film back as the darkslide seemed to provide sufficient protection. Although, I usually didn't bother to dust off the back, which may explain some of my dust-on-negs issues with my old 645pro.
I was just referring to the moment of changing. I hadn't thought about the static in the plastic and that being a dust magnet. I could make a bag of sufficient size for my hands, camera & backs to make the swap in. Sil-nylon is my preferred material, but that may be no better than plastic.
Two bodies could be an option for hiking. When backpacking, other than weekend trips, my stripped down 14lbs of photo gear nearly equals the weight of all my other gear combined. On weekenders, it's just silly and I am thoroughly ridiculed by my buddies, so why not another body.
