Wouldn't it be cheaper to make a proper tandem and a single rather than that mess?
If you wanted to steer from the bullhorns you could have strings or a linkage to the steering bars. It wouldn't make it any weirder than it already is.
edit:
And I hope that extensible tube is keyed or ovalized. I can imagine some unpleasantness if it's round and it starts to slip.
On second inspection it looks like it would make a lousy back of a tandem because you'd be counting on the top tube for all the stiffness (lateral and vertical). It doesn't look like it's designed to bolt a downtube on the front of the BB.
Last edited by bitingduck; 09-09-17 at 08:19 PM.