I don't see how you could fit all that in a messenger style bag. Nor in a shoulder bag if you want to work out of it.

I have a 2x3 Century Graphic in largish bag (with shoulder and waist strap). Along with the camera, I have roll film bag, meter, film holders, odds and ends, and can fit an extra lens on board if compact. I just don't see how you could also fit a SLR with 2 lens unless perhaps in one of the really huge shoulder bags. Certainly not in a messenger style bag.

Century Graphic bag 1.jpgCentury Graphic bag 2.jpg

So I would go the backpack route. If you don't want a backpack, then I would consider 2 separate bags each appropriate to it's own "system" (CG or SLR). Just my 2 cents.