Well for what its worth the philosophy is more or less as has been pointed out previously but its along these lines.
If you have an evacuation without an engine fire, you fire one bottle into each engine -- just in case.
If you have an engine fire on the ground I'm sure your QRH will tell you to fire one and then after a prescribed period of time (probably 30sec) the second.
If you are doing an evacuation with a fire you should have already fired both and hence none left and you will be going for a quick trip outside. Which makes the ensuing Evac checklist redundant as far as the bottles go because there's none left, (assuming you only have 2 on the 757) but then if checklists where written for every occasion we would have a New York telephone book.
Hopes this helps but I'm not familiar with the 757 but I am with other boeings