OP: Best option is to find a good used tandem (really not very hard to find at all) and buy it. Then put the current one up on ebay or craigslist and flip it, hopefully for near what you paid for it.

Other option is to fully disassemble the current bike, replace the frame and then replace each of the components one at a time to end of with a good used tandem that cost three times as much!

Apologies for being snippy, but facts are what they are. Try to avoid throwing good money after bad...so to speak.