Getting in between another teacher and his student
My first question is, why are you (the faculty) both teaching the same class at the same university without talking to each other?  I can understand the cultural differences, but this is even more of a reason to talk to your colleague about the class!

I would suggest (as posdef does in his comment) to talk to the other instructor about the class, offer to have him sit in on your class, and ask about collaboration.  You could also ask to sit in on his class if you'd like, but you need to be careful to phrase your request so it is clear that you're there to gain insight into his class and not to provide criticism.  I can't tell from your comment, "...if one teacher might be less strong in the subject than the other" if you believe he is the weak one or if you are, but regardless, you can both learn from each other.

Finally, it is in the best interest of both classes of students that you collaborate with the other instructor -- if both courses are listed as the same in the course catalog, the students should be able to expect that the class will cover roughly the same material.