Official FAQ wrote:
Stacking
Q: If have a Battle Droid within 6 squares of General Grievous, Supreme Commander, and I have a Battle Droid Officer in the same squad, would the Battle Droid get +4 or +8 to attack? Should these bonuses stack?
A: The Battle Droid would get +8 to attack in addition to getting Double Attack from General Grievous, Supreme Commander. These bonuses do indeed stack. The bonus provided by General Grievous, Supreme Commander is from a commander effect, but Fire Control is not. (If both bonuses were from commander effects, they wouldn't stack; you'd just get the better of the two bonuses.)