Side mount BCD is difference from back mount BCD, I am not sure whether did I understand your message correctly, if you just hang the tanks at the side of of BCD, it is not really called side mount. Side mount BCD has special rings near should blade to pull the tank close to body and there are hooks at the Bartok to hock the tanks. For side mount, you can use 2 AL40 tanks or 2 AL80 tanks. It is not common that dive center have AL40 tanks. it will not be convenient to bring your 2 tanks with your dive trips. And filling the tank will be an issue. Unless you are going on LOB.
Not sure dive center provide 2 tanks without extra charge or not?
I like to dive side mount because I can carry extra tank, and the group who dive with me will be happy to know that I carry extra tank. Usually I keep the right (second tank) at 100Bar, usually do not need to recharge for the whole trip because I never get to use it, my main tank is usually enough.
Come visit my booth (Andaman Tritan) during ADEX, we can discuss about it, and we also conducts side mount courses during our LOB.