You can connect the thing up in a few ways I guess. You can fill it via overspill from the main tank. You can use a pump to push fuel back into you main tank as it drops. You could Siamese the tanks at the bottom to be one big tank too. I did the full factory install on mine with the change over solenoids but used two fuel gauges. You can trigger the dash gauge to swap too, when you switch over, but that's not what I did.
Keeping the two tanks totally separate was my plan so that if I holed one, or maybe had bad fuel etc, I could run entirely on one tank or the other. It's quite bit of work, but worth it I thought. I generally run off the back tank first just to get the weight off the axle. Effectively the main tank is then the reserve if you like and we run only on the sub tank, filling than whenever empty. Doing both Morocco and the Baltic, I can't recall ever getting close to running out. Unless you are crossing the Steppes, I really don't think that you need to go bonkers with fuel capacity. It's all extra weight and all in a very poor place to be carrying it. Better maybe to take out the main tank and put in a bigger one between the axles.