Natalie:

Worksman puts out a folding tandem but it's VERY heavy.
Suggest you be on the lookout for a used Bike Friday (TwosDay) folder or a Montague folder (no longer being made but some used ones available). The Montague (yes, did a hundred+ miles test ride) has 26 inch wheels, comes in one size only (21x19 to our recollection), has upright bars and folds/unfolds in about 15 minutes and will fit in the trunk of a car. They referred to the Montague as a Tri-Frame, which means it folded down to 1/3rd the size of the full length. Also came with an extra $$ hardshell travel case.
However, with a little doing, most non-folders can be transported in a trunk. Take off both wheels and you may have to remove seatposts (with saddles and stoker bars still in place). Even if part of tandem sticks out of the trunk, use some padding, pull trunk lid down and secure with bungee cord. So a folder may not be necessary and no more work to do that than 'fold and stow'.
If you have a hatchback, pick-up, mini-van or station wagon, it's even easier than 'trunking'!
Have hauled our non-folding tandems in everything from a VW squareback, van, pick-up, Honda hatchback and a station wagon!
Hope this info helps!

Pedalon TWOgether!
Rudy and Kay/zonatande