Cutting a double does not weaken the rib/barrel joint. Everything that is joined on uncut barrels is still joined after the cut. The space between the barrels is hollow.
They do look unfinished if the space is not plugged. The traditional method of doing this is to plug the void with steel wool, and drip molten lead solder into the void. Excess solder is then filed flush.
The more expedient method is to use colored epoxy.