I think the original question was not whether duet systems were better for accompanyment, but how, if you've got a duet, do you use it. Personally, I tend to play the melody on the right, and use the left to fill in chords, bass runs or whatever seems right for the song. Fortunately, when I started playing, there was no-one around to tell me the duet was only supposed to be for slow songs, so I use it for everything. For faster songs, I use a vamping technique on the left, alternating the lowest note of the chord with the other two (rather like a melodeon).
Best way to learn is to simply 'twiddle' up and down in one particular key, finding what chords/bass notes whatever fit best. Then do it again in another key. I suppose it counts as practicing, but for me it's just relaxation. Now, I find that whatever I'm playing on the right, I can usually find a left hand part without thinking about it - occasionally I have to over-ride my instincts and teach myself to play something else, but not often. Sometimes I play something really clever on the left, and have to stop and work out what I did.