The rear of my longbike uses it's bottom bracket spindle to mount itself to the rear dropouts of the lead bike... it was a symmetrical spindle so there was no offset to worry about and it fit perfectly.
Both bikes have vertical dropouts.
My centre bar is 1 inch steel pipe that is bolted in place.
I opted to lower things a little to get a better chain line, bring the top tube down a little so that the bike would accommodate more riders, and it creates a slacker head tube and frame angle which makes the handling very neutral.