Originally Posted by
Car RAMROD
The only potential issue here (depending on a/c type though, I'm not familiar with all Boeings) is if you have fired both bottles into one engine, then get into the evac checklist and the other engine has caught fire too in the mean time. You've now got no bottle for that one. Incredibly remote possibility, yes, but if the other eng hasn't caught fire at this point you'd be firing the 2nd bottle into the one that was originally on fire anyway, presuming it still had the fire warning.
Two bottles total. One would use both bottles and not consider saving one for the other engine in a case like this.