At the same time yes, but for them to both work together simultaneously, no! WiFi always take priority over mobile data but if your mobile data is enabled then the android system can immediately switch back to it if your wifi has no internet connection.
Are you still using data when on WIFI?
Generally, when you are on wifi, your phone does not use cellular data. However, there may be some apps that use cellular data whether you are on wifi or not. Visual voice mail on an iPhone is such an app.