Deciding which tank you run off and use will effect your trim, and overall how the boat will ride. The main tank on most models is located closer to the boats center of gravity, giving it a better ride angle, as well as helping it run through waves. Since all grady's have engines far aft, moving weight forward is preferred, on inboard powered boats, engines are mounted lower and further forward, so fuel can be places further aft with less of an effect. I cannot be certain on all Grady models, but on most, you will get better performance numbers running your main full and your aux only when you need it, although i recommend using the aux from time to time so it does not sit dry or have fuel sitting either. When i use both tanks for long runs i burn the aux first. Grady labeled these tanks in this manor because each boat was designed to perform better off the main tank. Can you run an engine off each tank, yes, but I just think it makes fuel management a pain, watching 2 tanks at the same time, and my particular boat runs best off the main, and with fuel prices, I like to save wherever I can. You will not harm your boat or engines running off one tank over another. For each model, you should try and run off different tanks, try one then the other, see how your boat performs better and run it that way, but in most cases it will likely be off the main with a light load in the aux. I always keep 1/3 of a tank in the aux as a safety net, even when inshore and when I know the bite is local, I will add fuel to the aux and run it for a handful of trips just so my gas does not sit and my fuel burns will not be significant. I would not recommend running off both at the same time, not that it will hurt your engines or tanks, but just that judging your fuel is harder to do.