mmmm Good question
I dont know if there is an easier way
but personally id use Two invisible helper sprites connected to the touch function
On touch spawn invisible helper sprite 1 at Touch pos X,Y and record this pos data in a Temporary variable
and then somehow detect touch 2 and spawn second sprite at second pos X,Y and record that data
then make an event loop or normal event to check to see if both sprites are on screen if so...
then check for "distance" movement between the two., x, y pos data ..if the "distance" is increasing ..zoom layout out
if its decreasing ..zoom layout in...
if no touch on the screen destroy sprites and clear any XY data stored
C2 does supports Multi touch which is what you want directly....you could read up it here..
Hope that gets you started somehow