I don't have that hub so can't be completely sure, but based on other tandem hubs I have used (Hadley, etc.), my guess is that there is a spacer in the endcap that can be removed to make space for the outer plate of the drum brake. See arrow in my attached photo. Basically, what you are holding is actually two threaded pieces that screw together, with a removable washer/spacer in the middle. Use two wrenches (adjustable or vice grip) to unscrew the two parts. Install the first part of the drum brake onto the hub, thread the right-side piece of the endcap onto the axle, install the outside part of the brake (the shoe plate) over that endcap piece, and then thread the left-side piece of the endcap on to hold it all together. Depending on the diameter of the endcap space (where the washer-spacer currently is), you may need to drill out the hole in the drum brake. Arai actually made plates with two different size holes to accomodate different hubs. Hope that makes sense, but I'm pretty sure unscrewing those two pieces that comprise the endcap will solve your problem.
