Are the phones running ios7? If so you can go to Settings>Cellular to determine exactly where the data is actually going to. In addition, you can manage switches to each and every place in your phone where data can be used to prevent usage from certain aspects of the phone except on wifi. In addition, if you have mobile share and you are approaching your data limit you can go online to turn off data services to one or both phones to prevent the data usage from meaning a higher bill for you.
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.