Hi!
Glad you are loving the WBT with one class.
I would try to have each class compete against themselves, rather than against one another. The biggest joy is to beat your own records. WBT suggests the competition should always be with yourself - not against someone else - in this case not class vs. class. Chances are the "good class" will always win, thus defeating the egos of the other class, who may give up trying. Each class is different so utilize the WBT techniques that will work for each specific, individual class.
Next, find the reward that really motivates your other class. Maybe it is mind soccer, or the crazy professor reading game. Maybe smaller rewards can be earned by the end of each class, on top of the big reward on Friday. Also, get a timer and really utilize it to work on the tough rules.
Good luck:)