I carry one spare magazine on the belt at 9 o'clock for the primary gun and two speed strips in my shirt pocket for the back up gun.
Speed strips also carry nicely in the watch pocket of most jeans. Carrying a speed strip in the front pockets with keys or change is not a good idea, you'll wind up with dented cartridge cases.
I'm more inclined to draw the backup gun first, then transition to the primary (if necessary) before even considering a reload. I know that's backwards, but the backup draws a little faster than the primary.