Watch the Bmin7 - Amaj7 online guitar lesson by Robbie Calvo from Modal Expose
As you can see, all I've done to our chord progression is reverse the order in which the chords are played. Listen to the track and you'll hear that the chord progression now resolves nicely to the B minor 7 chord, the II chord in the key of A. This means our chords are from the key of A but have a tonal center (resolution point) of the B minor 7 not the A major 7. This means we can focus our licks on the resolving tonality and play the B Dorian mode and the B minor 7 chord arpeggios to improvise over this progression. That's modal playing. It's where the chords resolve to. Once you figure that out you can apply the relevant mode from that key. Of course you can outline both chords if you want to as well. There are no rules to say you can't do something but this is a great way to approach modality.