I've seen a couple of converted tandems, so it can be done.
You will need a motor with a bit of poke and biggish battery to match.
In turn, this means you will need to keep an eye on all up weight.
Much will depend on the weight of the rider and stoker, and how much luggage you want to carry.
The extra dead weight will make your tandem descend even faster than it does now, so you will also need to consider if the brakes will be up to that.
Apologies if I'm teaching granny to suck eggs, but braking such a rig can put a lot of strain on the forks/headset.
This can usually be dealt with by using the back brake much more than you would on a solo bike.
As regards getting the job done, Cyclezee has suitable motors and batteries, is known to like a challenge, and he enjoys a good reputation on here for customer service.
http://www.cyclezee.com/