This is generally how I'll see Joey preen his wings, usual pose, two feet on whatever surface/perch he happens to be on at the time.
Occasionally, I'll see him lift his wing like so, although he'll generally move his head towards the back instead.
But then things got weird.
Look closely at the picture below; his right foot is holding some flight feathers as he's preening them...
..then he got in a little issue; wanting to continue to preen yet not sure what to do with that foot that is in the way.