I think the OP meent something different, if you have to wait 30 seconds before you discharge the first bottle. The checklist covers 3 scenarios( actually four if you count the engine overheat light that directs you to this checklist if the overheat light does not go out), engine fire or severe damage or separation. In the last 2 you might not have a fire warning light.
If you have an engine fire, after you pull the engine fire switch and the the light stays on(which it probably will), you immediately discharge the bottle. Then, after 30 seconds, you discharge the second. You don't wait for thirty seconds after pulling the fire switch to see if the fire goes out by itself before discharging the first bottle.