I know this is an old thread but I'm new here. When I was a kid me and my sister used to ride around side by side in the back of my dad's Super Cub. Back in the 70's of course no one worried about if something like that was legal, but it was no problem.
Well a couple of months ago I did the same thing with my 7 year old and his school mate. Being the 21st century we were all concerned about it and my kid's mate's father used to be a flight crew seat design engineer for airbus. We discussed it and had the kids lean forward and see where their heads would hit in event of a sudden stop; they wouldn't hit the front seat. (the shoulder harness couldn't be used of course). Head strike this is called in the airline industry; always an issue with passenger seats. The rear stick comes out in the Super Cub, so that issue wasn't a problem. We never discussed a single belt over the two occupants, indeed I've thought this wasn't really a big issue.
The Citabria has more room than the Cub (as any Citabria owner will be quick to point out), so if there is a seat that is full width it sounds like is shouldn't be any problem. Does the rear stick come out in the Citabria like it does on the Cub?
Also here in New Zealand we have a three seat Super Cub (local mod) where two adults can sit side by side in the back. The top longerons (bottom of window on Cub) are pushed out and a wide seat installed. Again, no issues regarding stick clearance and that had a single belt across both occupants. I had a ride in one when I was a youngster and remember I couldn't make up my mind where to sit; on one side or in the middle!
Cheers,
Andrew.