A agree that two systems is a lot, unless you have a specific reason for taking both, like you generally want to shoot MF, but want the option of fast lenses or long lenses, and you'll use both systems enough to justify it. Taking too much stuff can sometimes get in the way, and I find I often do better just to stretch the capabilities of what I do bring, and having everything in the same format creates some unity of form in the images I bring back from a trip.
If I'm traveling with my Bronica, I may bring 1-5 lenses, depending on how much time I think I'll have for photography, and what I think my photo opportunities will be, but if I bring a large kit, I'm usually not carrying it all the time. I'm more likely to carry the camera on a strap and maybe one or two lenses in the pockets of my overcoat or in a small bag if it's not overcoat weather.
I don't like to travel in cities with a photo backpack, because it gets in the way on public transportation (you have to take it off to avoid whacking fellow passengers when you turn around), and it is a very easy target for pickpockets who can liberate items from a backpack without you noticing until it's too late.
If you want a shoulder bag that can convert to a backpack, you might look at the Think Tank Urban Disguise bags that look like ordinary laptop cases that have the option of a backpack harness for when you're carrying the full kit on and off of the plane and such. Some people think laptop cases are also likely targets, but they are so ubiquitous now, that I don't think they are as much as they used to be, particularly since most laptops aren't worth stealing unless they are very new and clean.
. Fits as an airline carry-on too, just without the gun.
