Why don't you go to fleebay and search motorcycle shocks something like a Vulcan 500 or other 500-750 almost any could be made to fit, a bike shock would have a stiffer spring. The downside to doing this is all that extra weight you might add could stress the frame and cause it to bend or crack. A single shock puts a lot of stress on the frame......I know I bent mine. 
Yep, looking at various scooter brands for a shock, and of course motorcycle shocks. The LIKE would use something like a vintage bike shock (old Honda 750's, even 1000's) Mounting at top is 10mm, bottom clevis mount is 8mm. Many scooter shocks have this, very few motorcycle shocks have these measurements. I have been on the web for 2 1/2 mos looking and placing phone calls.
The biggest issue is that 2% of shocks have spring load ratings listed, spring width, and/or eye to clevis distances listed anywhere!
It requires finding someone to pull shocks off a shelf and measuring them in the warehouse - good luck with that!
OR, finding someone who will tell you that their shock works great with 2-up riding AND how long their shock is. good luck with that!
VIVO gave me the only response so far!! Good man, Vivo is.
