I have 2 kids. when my second was born (July 18th, middle of a PHX summer) my first was 15 months old. To be honest, I avoided going out anywhere with the 2 of them after 11am because it was just too hot. As far as ahte carrier, I don't think there is a suggestions. Most carriers are made of a vinyl like material, which causes the baby to sweat. So unless you can buy a new carrier made with more of a cotton type material, I say avoid going out after 10-11am in the morning with both kids. And if you absolutely have too, take the baby in a carrier/baby wrap instead of the carseat/infant carrier.
Good luck and welcome to the oven we call summer here in PHX! And in case no one told you....it lasts until October out here (sometimes even in Novemeber we are still wearing shorts)