I think Roland is the only Arranger that has not had a dedicated multi-keyboard organ designed for it
Bill (Abacus)-- Roland has the Atelier series, and especially the AT-350C which competes with the Electone STAGEA, etc. I don't believe Korg produces a self-contained two manual arranger though.
I had a few years of piano lessons in elementary school, but we only had an unweighted single-manual organ at home. It had neither the sound, nor the heavy rewarding feel of my teacher's piano. I struggled with the meter of unfamiliar songs in my lesson books, and with syncopated changes in the left hand. Once I discovered automatic organs with a reasonable repertoire of chords I never looked back. My only regret is that I never learned to play pedals.
I don't gig out. For a while I had two arrangers MIDI'd together on an A-frame stand. I might even go back to this setup. One advantage is that you can use a MIDI file AND style at the same time, or even drive the style from a channel in the MIDI file. One board can be set to recognize chords in any inversion, while the other is set up for 'on bass.' I hate turning pages, so Yamaha's "follow the bouncing ball" rolling score display was another plus (although I'm told there are notebook and iPad apps that do this.)
I agree with Rosetree--I would insist on having real MIDI jacks. I don't know of any workaround to connect USB MIDI directly to another instrument.
To my observation all of the improvements to arrangers since the late 1990s have been in the quality of the sound samples (which is a shame, because this could have been accomplished through VST's or MIDI sound modules like Ketron's SD-1000.) Think about the core feature set which defines an ARRANGER: chord recognition, style programming and behavior, etc., have not advanced, and I think there is plenty of room for improvement.
With today's tendency to release "patchware," it will be years before this kind of integration is realized reasonably bug-free in a single machine. My $.02.