I do it all the time; for whatever reason, we've had several flights when myself and Mrs. Pokeable are split (both FO), but I get the battlefield upgrade.
We board together, but I show the F flight attendant my boarding pass, and let them know I am sacrificing my F seat to my wife. Almost always accompanied by a smile or comment by the FA, and then I take my wife's window seat while Mrs. Pokeable sits in F.
Have done this 3 or 4 times without problems or incident.