The Co-Motion Periscope would be a very good choice as it comes with 26' wheels that would allow you to run a wider tire for the limestone trails. Frame size 18x11 with loads of seatpost adjustability; around $3,000.
Also look into the Burley mountain tandems that come in a 19x15 size, at about $1500. Or the Burley Zydeco Mixte model at about $1150. Both of course run 26" wheels.
If traveling by plane is in the future, a collapsable Bike Tuesday (by Greengear) is another option. They run 20" wheels. Don't know the price offhand.
All 3 companies mentioned are located in Eugene, OR.
Good luck!
Rudy & Kay/Zona tandem