I suspect we will be doing the same thing soon. How I am going to do it:
Our second tank will be very close to the first tank, similar to yours.
I am going to put a Y in the fuel
. This Y can be selected for tank one or two. This is easy stuff.
I am going to put a T in the fuel
vent to select between tanks. SO when you select Tank 2, you also change the vent to tank 2.
I am going to T the returns and supplies. This again is easy to do. With teh valves, I will be able to choose which tank to pull from and which to deliever to. If I were you, I would run the Filter Boss, if that is what yo uare going to use, between them so you can effectively filter between tanks as well as out. You could even put a manual pump
inbetween them so that you could run the Filter Boss without having to run a engine. THis allows you to polish your diesel without running the engine. THis is popular amongst trawlers and larger motor sailors.
THe RIGHT way is to run new everything, including the deck fill and vent, but I am too lazy to do all that and do not get excited about another hole in my deck.