What about 1 and 2/3 Delta trikes? Hitch the 2/3 (no front wheel) one to the rear of the other. It can be removed when not required. No steering required and the 2/3 can be made exactly to suit your needs. I participated in a night ride recently where there was a couple with with a disabled son. There was Dad on a full Delta in front, followed by their son on a hitched 2/3 Delta, followed by Mum on a hitched 2/3 Delta. The young fellow, otherwise unable to participate, had a whale of a time on the ride. I hope to meet up with them again.