As has been stated, the Halon 1301 used is a gaseous extinguishing agent. It works by being heavy and inert: it sits around and prevents oxygen from reaching the fire while absorbing the heat. Very little of it should chemically react.
The nacelle has been engineered to maintain the proper concentration. It should work a bit better on the ground since you're not moving and the atmospheric pressure is higher. Since its 5 times denser than air, it will stick around for minutes.
If you fire the second bottle too early, all you'll do is push out the existing Halon to the atmosphere and waste it. Another concern is that because how Halon absorbs heat, when it touches hot components they will rapidly cool and cause thermal shock and major engine damage.
I'd say to go ahead and pull the handles, but as the checklist says, pilot judgement is necessary before the first bottle. Wait 30 seconds for the second bottle, but evacuate in the meantime if necessary.