2 phones one family plan but two data plans?

Wife and i each have an iphone. We are on new $109 family plan. Guys at apple retail store say thats all we need that data plan covers two phones only extra if hit three phones then need another $20 data for third phone.
Pamphlet at store seems to back them up.

At&t phone reps say we need seperate $20 data plan for each phone!

Thoughts? Who is correct? Who should i speak to if apple guys are right?