Page 3 here - http://s3.pexsupply.com/manuals/1284398486438/39050_PROD_FILE.pdf
you will see heat first stage is W1 and 2nd stage is W2
Cool first stage is Y1 and second stage is Y2
Then you will see the other necessary terminals for R, G, O etc...
Doesnt your economizer have an enthalpy control that Y1 runs through? Then at a given set point...it changes from free cooling to mechanical. An economizer will not be considered a stage because it then switches itself to mechanical on its own.