2 better than 1, but...
2 subs give you better integration, as Yamahaluver stated, as well as higher output levels. If the 2 were smaller drivers than the single, like two 8" vs one 10", than you won't have as good low frequency extension. I was just talking with some customers eariler today about this situation. In your case, I don't know how much the 9" differ in extension than the 10" you have, so you might want to look into that. I'm talking about actual, practical bass extension here, not the claim specifications that are provided by the manufacturer. Too often they are inaccurate.
Good luck.