I have no idea what size a 750xxl is, so please just post gallons or dimensions for better help. This is like telling me that you have a n3s2 motor in your car instead of just saying a honda v6.
The time to use one is when you want to. There are other methods that are plenty good (kalkwasser, 2/3 part, etc) and work well, so you need to decide what works for you. Each have their pros and cons - most people unanimously choose calcium reactor with larger tanks with large demand.
You do not need a second chamber if you tune your reactor well. If you are using too much co2, then a second chamber can help some to get the effluent pH up. If you want to use a ph probe and controller to run your reactor, then you will likely need a second chamber since they are not efficient.