Couple replys:

She is a Vet student and so taking more then one day will be impossible as far as I can tell.

Why would your butt get sore on a tandem? Would a sprung saddle relieve some of this?

I ride over 100 fairly often in just plain shorts (no padding) on my brooks champion flyer with no saddle sores. Im 27 and most of my friends make fun of my sprung saddle but I LOVE it so I dont care.

The more I think about this the more I think Im dreaming a bit. I would love to make that trip with her but I think we are left to making it on 4 wheels until she gets out of school and has more time to ride.