Revisiting Dual Compressor Simulation for Logixpro

First i would like to say that this is a grat site. I was viewing the thread on the Dual Compressor Simulation for Logixpro. I have run 150 mph into a brick wall on this one. I dont understand how Exersise 2. Is it possible to toggle the compressors without using the second pressure switch? i normally would not want hints but i am progressing through the simulations and for some reason i cannot even get started on switchiing between two compressors without timers or counters or the second pressure switch . Seeing as how this topic has allready disscussed i understand if yall just redirect me to the previous thread. Kudos to all the pros here who give advice on this sit, i am really impressed with your involvment in this industry. Thanks!!