The amount of time the Halon has to act on the fire is more important than the quantity, and discharging the second bottle too soon may just result in pushing out the Halon present before it has had to to suffocate and start its job.
That is a very relevant technical comment and surely one which the boffins at the manufacturers should specify. It is a specialised scenario better known about by the pyrotechnic chappies than airline managers. Indeed, via 4 different members of the B737 family, B757/767 and 9 operators I've been told every which way to behave in this supposedly straight forward scenario.
Are there any knowledgeable fire engineers out there to comment?