Students were usually introduced to fixed step double push hands once they started learning the long form. As if one hand pushing wasn't complicated enough, we now had another hand added into the equation.
The type of double push hands we learned is the same as in the Ch'eng Man Ch'ing style. The style is not really important;what is are the principles being explored. Typically these are the four sides, ward off, rollback, push and press. The four corners are pull ,split, elbow and shoulder. Together with the four sides these make up the eight energies along with central equilibrium ,gaze left and right, step forward and back. In total these are the 13 postures or strategies of Tai Chi Ch'uan.
These 13 postures / strategies are in pushing hands and the forms. The practise is about understanding these. The understanding gained from pushing hands feeds back into the form, from where further insight feeds back into pushing hands.