Well, it's two moves relative to the centre column, but it's a single move relative to each of the outside columns. I was just interested in whether the rules are 'physical' or 'theoretical'.
Certainly with a good cube you _could_ hold the outside and push to twist a center slice. From www.cube20.org -- "We consider any twist of any face to be one move (this is known as the half-turn metric.)"
Also, "New results: God's Number is 26 in the quarter turn metric!"
