I sometimes get the shanks. When I do they stay with me for awhile. A drill that an instructor taught me (also to help with my flat swing plane) was to put three golf balls down a few inches apart and hit the center ball (to help with focus). This works a lot for me.
My question - When playing a round, is it legal to put put down two tees with a golf ball in the center, either on the tee box or fairway? Would this be considered an aid?

not legal...tee up next to something already there. Just don't put something down after you tee the ball

Part of Rule 14-3 addresses this issue...

14-3. Artificial Devices, Unusual Equipment and Unusual Use of Equipment

"...Except as provided in the Rules, during a stipulated round the player must not use any artificial device or unusual equipment, or use any equipment in an unusual manner:

a. That might assist him in making a stroke or in his play; or..."

Hope that helps.

Great drill, but sorry, can't do it on the course.

HOWEVER, nothing is stopping you from teeing up between divots, next to the teebox markers, existing tees in the ground, etc etc etc. Just as long as you didn't put it there.

I do this ALL the time. Use existing marks for alignment and clubhead path. Just like picking a ball mark/spike mark on the green to hit your putt to.

Thanks for the tip... I can't believe I couldn't figure that one out on my own.