Very few tandems will paddle well solo. Adjustable seats are certainly
better, but consider buying two boats, or renting if either use will only be
occasional. Any convertible boat will be a compromise.

That said, you might want to look at the Seda "amigo" I haven't paddled one,
but it looks like an interesting idea. Of course, for the price, you could
probably pick up a used plasitc tandem, and a used plastic single. What
uses do you anticipate?

Geoff