Btw, I totally hear you on the “narrow the battlefield” point.
But let me make my pitch for two Xjams, because I actually think they make the instrument easier to learn and play, despite looking more complicated.
The key thing is symmetry. With two Xjams laid out next to each other, each hand basically gets its own hand-sized playing surface. You don’t have nearly as much weird crossing over, reaching across the other hand, or approaching the same little pad from different angles. The movements become much more regular.
There’s also a cognitive advantage: rather than one crowded controller where every bit of real estate has to do multiple jobs, you can give important functions dedicated pads. That’s how I handle ghost notes, for example — I don’t have to reliably produce an extremely low velocity on the same snare pad; I have separate pads mapped to quiet snare hits. And because I have enough space, I can even have more than one ghost-note pad so I can roll between fingers.
So although there are twice as many pads sitting in front of you, there are fewer rules. Each hand has a predictable region, the layout can be strongly symmetric, and you can solve things with spatial mapping instead of increasingly precise finger/wrist/arm control.
It also feels quite close to the general logic of the FGDP layout — two hands, roughly symmetric regions, dedicated places for the important sounds — rather than some completely bespoke monster setup. Tailor made for alternating hands, but effortlessly accommodates both independent hands and other techniques, such as Rob’s innovations on rudiments in his Next Level Beats course.
And since Xjams are relatively inexpensive, two of them are still competitive price-wise.
Obviously it’s amazing and awesome to master a single xjam. I just wouldn’t necessarily think of two Xjams as the “advanced, more complicated” version. In my experience it’s absolutely the opposite: more physical surface, but a simpler instrument to operate (no crossing over) and control (e.g. velocity control).

-David