In your existing program you have a bit that is used to turn the compressor on. I don't know what you called it, but for the purpose of discussion let's call it "Run_Command". You will need to modify your existing logic to add a bit that I'll call "Use_Compressor1". You will also need to add a similar rung to turn on the second compressor, but in that rung you will need to use a bit called "Use_Compressor2" instead of "Use_Compressor1".
Now your challenge is to create logic to control the two new bits. That's where the counter your instructor suggested could come into play.
For example, each time you start either compressor you increment the counter. When its .ACC value is even, turn on the bit, "Use_Compressor1", when the .ACC value is odd, turn on the bit "Use_Compressor2". I'll leave it up to you to figure out how to determine if the counter's .ACC is even or odd.
