You are referring to "dual force options". The current school of thought is to have a firearm in your strong hand, and the pepper spray in the support hand. Use your THUMB for the pepper spray, and your trigger finger for the firearm. You are less likely to get confused, but a bit of practice is always a good idea.
Pepper spray and a gun make a good combo IMHO (never having to have had to use either in a real-life self-protection scenario, I'm only speculating).
If the pepper spray holds off the attacker, that's fantastic. If it doesn't, and you are forced to shoot, your pepper spray attempt may help from a legal standpoint should your actions be questioned in court. You attempted a non-lethal defense, but the attack continued. You were then forced to shoot.