Watch the I vi: Area 3 online guitar lesson by Brad Carlton from Guitar Lab: CAGED Comping
This lesson covers the I vi progression in the key of A in area three. The chords are A and F# minor. The D form from the CAGED system is used for the A chord and the C minor form from the EDCAG system is used for the F# minor chord. The respective modes for each of these chords are Ionian and Aeolian. The formula for the Ionian mode is 1 2 3 4 5 6 7 and the formula for the Aeolian mode is 1 2 b3 4 5 b6 b7. The A chord is spelled A C# E and the F# minor chord is spelled F# A C#. Any two triads a third apart will share two common tones. These two chords share the A and C# notes.
The arpeggios for these chords are labeled in red in the diagrams. The remaining four tones are labeled in black which then form the modes for each of these chords. Each of these chords have a pentatonic scale which is an extension of the arpeggio. For the A chord, add the 2 and the 6 to the major arpeggio to form the second string rooted major pentatonic scale. For the F# minor chord, add the 4 and the b7 to the minor arpeggio to form the second string rooted minor pentatonic scale. As you can see, the same pentatonic scale form serves for both chords. This is a rule: any two chords built from the major scale which are a third apart will share the same pentatonic scale form.
Be sure to memorize the formulas for your modes because then you will clearly see the pentatonic subscales that exist within them.