- firing a bottle does nothing to the fuel supply.
There is no time restriction on firing the second bottle. Evacuation - that is what you are paid to decide.
Actually, I do not know of a modern jet that will allow you to discharge a bottle W/O first pulling the fire handle thus cutting off fuel.
First possibility you should think of is a blown duct, as you reject after engine idle if the warning goes away, fire handle pulled as said before visual confirmation (window,cc or atc). Blow your bottle as you wish and do not panic, confirmed fire may force egress before you blow your 2nd bottle. Good egress communications to cabin depending on nature of fire wind etc..
Teamwork and a good communication with your other crewmember(s) is vital.