1) Midiing together is not mandatory. Simply mixing sounds (output-loop effect input) is alright to have audio backing performance. If you use SD card with _fixed_ audio backing tracks -- this mean _not_ live performance.

2) Two-manual mean only 20cm _up_ in space. No wider neither deeper. And it no less portable then two one-shelf stand instead one double-shelves. It's like in organs: then you can have live performance with two keyboards simultaniously.