Watch the Bmin7 - C#min7 online guitar lesson by Robbie Calvo from Modal Expose
Can you determine what key a chord progression comes from just by looking at it? Yes you can and the only key in which B minor 7 and C# minor 7 can exist side by side is in the key of A major.
Our chords are in the key of A major but the A major 7 chord isn't present, so what is the tonal center of this progression? On listening we can easily and quickly determine that the tonal center is B minor 7. So what we'll want to do when improvising is play B Dorian and resolve to the chord tones of the B minor 7.
You can also outline each chord as they change if you are proficient with your chord arpeggios and that will sound really strong and melodic, just make sure that you resolve to the correct tonal center and its 'Sweet Notes' when the track ends.