I very rarely see a used Greenspeed Tandem for sale, but they do show up occasionally. Unfortunatly it's seldom at a convenient time or location.

Here's one that I just noticed today....followed a link from BROL to this e-bay ad. This one has S&S couplers.
https://cgi.ebay.com/ws/eBayISAPI.dll...ht_1256wt_1141

If nothing else, this serves as a price point for further reference once a decision has been reached regarding what, if anything, to shop for.