How would I stack promotion codes (PC)? Folks have discussed doing it in previous but pretty old threads. And it also looks like AutoSlash and their rep here have helped but their website said no cars (probably next time I need to try more in advance of my travel).

I'm USAA and also want their double upgrade (a PC). United MileagePlus has an additional PC for triple miles. Well, the Hertz website I see only has one textbox in the form where I can enter a PC. Thanks for any tips on how to enter the 2nd PC!