Two ways jump out to me. One is to do two gussets with a divider in the middle. The other way is to attach the divider to the front or back panel of the bag. That way you only have one gusset, but there will be 3 layers of leather at either the front or back edges. This divider can be tight and function as a slim pocket or you can allow a little excess width and it will expand up towards the middle of the gusset. I suppose a third way would be to sew a divider to the gusset, but you'd see the stitching line. Now if you are using a liner, you have tons of options.