I would go with the single 22.5 just for ease and simplicity of only having to monitor or control a single smoker, but that's just me. It all boils down to the plan that you feel most comfortable with. If you think going with the dual 18.5s is the better option then I truly believe you should go with it. You my man have options, and that in and of itself is a great thing!!
Good luck, and take pics, and keep us posted,
Tim