By adding the second batt on stock, the alt is then presented with the obstacle of keeping 2 batts up to charge. This may give advantage towards your amp, but puts more load on the alt.
Also put this in mind. A regular lead acid batt has a differ purpose. Even if it has high cca, that is used for 30 second bursts such as car starting. They aren't designed for discharge pressure from a stereo and such. You are looking for high AH and RC.
If you were persistent on using another batt on stock, a sealed will store charge easier, longer, and will be easier for the alt to maintain the charge.