For me, it depends on what I'm going to be doing with cameras that day. If I'm going out on a job then there's a whole bunch of stuff in the car, ranging from Bessa R, to Spotmatics with 8 lenses, a Pentax digital SLR (that accepts all my great Takumar glass), a Rolleiflex, etc.
However, if I'm just going to wander around, I take one camera with one lens. Invariably, it's the Bessa R with the 35/2.5. The nice thing about striding forth of a day with one camera/one lens is that it eliminates pondering lens choices, and especially with the Bessa R, being a screwmount beast, it eliminates wasting valuable time "screwing around," trying to change lenses in a hurry. Also, I don't like being burdened down with camera bags or pockets full of lenses and extra bodies when merely going for a stroll. I particularly like the Bessa R because of its small size. Fits into most jacket pockets.
As someone said earlier, with a single camera/lens, either step forward or back up a few feet.
Ted