Those frames are really set up differently. Are you sure they would both fit?
You can use this to estimate value:
http://www.thetandemlink.com/usedhome.html
The Santana is not a bad price, but it is pretty old (note the down tube shifters) and you could probably do better. On a bike that old you really need to check the elliptical and the other wearing parts to make sure they are in decent shape, otherwise you are buying a bike and a big repair bill. The Trek is a decent looking bike, but it too would need to be inspected. there is also the fit issue. Between the two (and I own a Santana so I am not biased against them, despite their weird rear wheel spacing) I probably prefer the trek.
Is this for you and Farrah and how do you plan to use it?