Subtract in increments until a column equals zero, then move to another column????

Not that great at excel, so bear with me... I want to create a production schedule that will show me the number of each of two similar products left to produce after a given amount of time. I'm measuring time in terms of total pallets left in WIP(I.E. 100 pallets to begin with, 50 left to produce at 50% complete). Both products take essentially the same amount of time to produce. One product MUST be produced first.

How would I go about having excel calculate product 1 being subtracted at a constant rate, and then have it move to subtracting from product 2 once product 1 equals zero? I'm trying to automate this as much as possible so that I can put in the number of each pallet type and let it run.

The ultimate goal is to use these numbers to calculate space left in a warehouse... The pallets are different sizes, thus the delineation of product 1 and 2.

Thank you!

Thank you so much! This is awesome... Now to figure out how you did it...

There is one issue looking back on it... The 12 pks shouldn't be used until all of the 18 pks are used, so the left side should say 44 all the way down. It won't switch over to the 12 pks until the 18 pks are entirely used up.