Details and pictures would help. I assume the girlfriends bike is a rear drive bionix conversion (135mm) drop space and you tandem' space is 140 or 160mm. If so, not easy to do. Also, rear drive tandems are really tricky if you ever need to ride it solo. Slip and spin.
Consider a front gearless kit (~$150) on the tandem and using her battery for either bike. The kit's 100mm axle spacing will fits all tandems. The gearing on decent road tandems is dialed in and expensive. He rear wheel would probably screw that up.
I've done dozens and find 36v500W adequate for a light road tandem with strong riders. 25 mph on flat and 2 miles/ah. It's also really easy to drop the front wheel (and all the weight) for transporting the bike on roof, etc. The stock wheel can reinstalled in 5 minutes. (although, I guarantee you will never do that)
Here's our Santana Arriva.
