I won't get many backers here, but I just ordered a Lamborghini Viaggio Tandem Bike from Amazon for $506. Gets good reviews on Amazon that mostly seem real, despite shill claims. Frame is solid from what I read, but many of the other components are so so, and over time many have upgraded to get desired performance. I plan to change out the brakes but will wait for a while to see if SWMBO likes it enough. Also plan to do some club ride with my son, and since we're both strong riders, we'll give this a work out.
Looked for some used bikes, but not much choice out there, not like "half bikes" as the TWOfer community likes to say. I figure that if we find we like it and stick with it we can get a better bike when we better understand what we want. For the time being, this will get us started.