Don't bother trying to "tell the arduino to read a difference sensor value" each time. Also, are you using a push-sensor? Or a push-button? If you are using a "push button", stop using the word "sensor."
Instead, read both buttons each time through the loop.
The problem with determining which was read last is the loop will execute hundreds or thousands or tens-of-thousands of times before the next time you push a button. Why does it matter which button was pushed last?
You're already tracking if you are doing an action based on the last button. Or at least, I think you are based on the really really complicated IF statements you have.