Ramrod, each engine has two bottles.
The SOP is to wait 30secs between bottles both on the ground and airbourne, firing them both at the same time is outside of SOP and of questionable benefit.
Fore those that don’t realise, the fire extinguishers only discharge between the engine structure and cowling displacing air, they don’t go into the engine core. One squib theoretically can displace all the air in that space (assuming the cowling isn’t damaged), firing both has negligible extra effect. The 30 sec pause is to allow the first extinguishant to take effect, when air (O2) starts to leak back in the second squib forces it back out again giving 60 secs of full retardant.