I'm no expression guru so I'm sure there is another solution out there, but why not have both of your scales driven by a seperate thing. Create a null object and apply a slider to it.
Then, on each one have it be their scale + the slider and on one of them add *-1 (put the slider *-1 in parentheses).
Then when the slider goes to 1, one of the layers will be plus one and the other would be minus one.
But, that's just one solution, I'm sure there are several others.