At the risk of going further off-topic... I have combined two style engines, but probably not the way you're thinking of doing it.
I'm very picky about playing a minimum number of keys to get the desired chord. IMO Roland's Chord Intelligence does this better than any other brand. Moreover, I'm used to it.

So I created a very special style for the Roland that plays sustained triads or tetrads and an appropriate bass note whenever a key is pressed. This way, I can play on the Roland using my preferred fingering. The Roland sends chord + bass "note on" messages via MIDI to my Yamaha (or any other arranger.) The Yamaha interprets the incoming notes as fingered chords, and styles play normally as they would from an external master keyboard. Sync start works as one would expect.

Technically I'm playing two styles at the same time. But my special style is like a "free play" style. There's no rhythm, so the tempo and time signature are irrelevant. I'm not sending MIDI clock, SysEx, or otherwise trying to sync the two arrangers. The Roland style just augments the single notes I play by turning them into full chords.

If I play a chord progression with appropriate timing, and do a quick record on the Yamaha, the resulting MIDI song can be fed back into the Roland to drive its style engine. This functionality isn't as convenient or "real time" as the chord sequencers featured on some recent arrangers.

I'm a big fan of modules. Maybe someday when the prices bottom out and they're not the latest thing, I'll get an SD-40 of my own!