I dont have shared data myself but what yo could do is install 3g watchdog on both phones https://play.google.com/store/apps/details?id=net.rgruet.android.g3watchdog&hl=en
it monitors what data thephone its installed on has used in both mobile data and wifi, it will also show you what application has used what amount of data. that way you should be able to see where the data is being used, and on what across both handsets.
I am surprised that it is stating that high data usage applications will be ran through mobile data, normally its the other way around, it will run through wifi, if wifi is on then all data should run through that, is it something specific to your carrier ???