I did this for a while with my Roland E-50 and Yamaha PSR-S950. I had them on an A-frame stand. Both keyboards on a slight tilt to promote accessibility, but mostly to permit a clear view of the displays.

I strongly prefer Roland's "Chord Intelligence" to any of Yamaha's chord fingering systems. I didn't want to relearn stuff that was committed to muscle memory. On the other hand, Yamaha gives you MUCH more control over the subtleties of how the arranger transposes the pre-recorded pattern. They also have the largest library of great-sounding styles. So I made a special style on the Roland that generates three- and four-note block chords on demand from Roland's simplified fingering. I output these triads and tetrads via MIDI into the Yamaha, and it drives the Yamaha style engine. (This concept can be applied to almost any two brands of arrangers if you're partial to one brand's fingering.)

The two-tiered arrangement permits use of the Yamaha's MIDI player and superior color VGA score viewer (think "follow the bouncing ball.") A song can be played simultaneously with, and tempo-sync'd to the Roland's style engine. Yamaha by itself does allow you to start a MIDI song and style simultaneously. But once you press a key in the chord area, you have to play the chords yourself for the remainder of the song! With one arranger as song player and the other for styles, if I mess up or lose my place, I can quickly unmute the recorded MIDI, and then rejoin live play at the chorus, etc. Fake? Yes. Effective? Very!

The Yamaha's multipads were also available to add some variety- this was a feature the E-50 lacks.

As you can surmise from the above, I never really played on the Yamaha keyboard- mostly just used it as a module. You won't come close to the ergonomics of a two-manual organ. The odds of getting two arranger keyboards at correct playing height, while retaining access to the myriad controls and displays are daunting. Even just keeping both displays in view was a challenge.

Ultimately I scrapped this setup in favor of a Tyros 5 / BK-7m combo. Mostly for cosmetics-my urban contemporary living room doesn't look like a music studio anymore! But I miss the two-arranger setup. For a less-skilled player, I feel there is definitely some synergy / additional benefit to be gained from MIDI'ing two arrangers.