Thoughts - if you set the crank up so the two "inside" feet were oppposite then you could slide the seat almost completely central and it would be easier to ride solo. You could also leave the original steering tube and arrange some kind of release for one of the handlebars to move it to the middle position again.

As a 100 KG person I'd be terrified of the cantilevered seat support snapping off, bending, or even simply rotating under me. Two longer seat tubes would provide more structure, but the framing would increase heel strike risk. As is, the rear of my inside thigh could touch the horizontal seat post/beam.

Who has control of the brakes/gears?

And a non-technical question, who is "legally" in control, should there be any problems?