You are not as free to stand on a tandem as on a single. All those little ups and downs you do without thinking on the single just are not there on the tandem (unless you are a really experienced team). You'll be working alot to plan out when to pedal, when not to, when to power, etc. Add in planning butt breaks and it can be very aggravating to lose so much control that you don't take as many butt breaks as you should. A sprung saddle could aggravate or help. Some people like the extra bounce some do not. Time in the saddle would tell. You can search through here on suspension seat post and see the variety of replies.

Ya, definitely get a few miles under your belt together on a tandem before putting yourselfs in a must do situation. Maybe bike part of the way there and have your ride meet you along the way? One key question to the plan, is she as excited about the trip via singles/tandem as you? No reply necessary to that question, I was just pointing out that I don't see anywhere in your write ups where you say she is as into riding as you are. I had a few instances where interest, capability and expectations did not match. Not pretty .

It sounds like you would do the trip one way there and get a ride back? Hmmm, silly question, you are car free, so that must be it.