First! I cut both shirts at the exact same place (they were identical shirts, different colors), a half inch below the sleeve seem
Then, just switcheroo the tops!
Add a strip of stretchy lace and pin the pieces back together!
A quick zig-zag to sew them together.
Then I top-stitched the lace down at the side seams.
Volia! Two brand-new color-blocked shirts that I love!
(Finished product pics to come... oops!)