Equal pairs are when both leaves of a pair of gates are the same width as shown here.

Unequal pairs  are when a pair of gates has unequal sized leaves. Often seen in 5 bar styles with a larger gate and a smaller pedestrian gate. The same thing can be done with boarded pairs to a certain degree. The smaller pedestrian gate latches onto the larger gate which will be fitted with a drop bolt. Note though that this doesn’t work with automation.