Kyle,
One suggestion you might consider. Check out some of the other tours, including Overlord and the one offered by the Museum in Caen. However don't book anything yet. You could also check out Ellwood von Seibold, however he may also require a minimum number for tours and the cost of his tours may be prohibitive.
In the same circumstances, I'd probably contact BattleBus again and ask them if it would be possible to add your name(s) to a waiting list. Provide them with the name and contact details for your Hotel, your Cell phone number (if you'll be travelling with a phone) and E-mail address.
As I recall, you'll be there in late March and it's possible that other people might request a tour by that time, so they may in fact have enough for a group. Of course there's no guarantees, so that's why it's a good idea to have the "backup plan" with the other tours.
Good luck!