I don't follow this. Why would you not be able to choose either single *or* dual binding posts? Why would outboard eliminate a solution with a single BP?
If you have an outboard xo, then you would need two separate wire runs from the outboard - one for the coax and one for the bass driver. Therefore you would need one pair of binding posts for the coax and one for the bass driver to connect the wires from the outboard. Otherwise how else could you connect the wires going to the drivers?