It is possible to be using both,  well for both to be on anyways, at that point there is no way to determine what is going through wifi and what is going through cell.  Just like if you had a laptop hard wired into your router and also were connected to your router via wifi. Only in your case, it's is your phone service provider that gets to tell you what you are using and how much.   I do not know what kind of phone you are using but I would suggest searching for a free app that turns off cell phone data usage when there is a viable wifi connection.  That would be the best option. Yes it can be done manually, but you def. seem the type to have something like that be automated for ya
Also, if you (general you, not you you boobear) use netlfix primarily on your mobile devices, go into your netflix account and then Playback settings, and set it to low, that reduces the amount data that is used while you binge watch. 4951