Hi Dave
The best way is to have the two tanks with valves fitted at the tank so that the levelling/drain line can be closed
As Rob says this would allow you to isolate a tank in case of a contamination or leak.
However what you need to establish is where your supply and spill return are connected. and if they can be directed to ether tank.
A diesel takes a supply from the tank, uses some of it to power the engine and some to lubricate and cool fuel pumps etc, this second portion then returns [More than most people think] to the tank.
With a permenently open balance line no problem.
Closing the balance line, not much of a problem, reduces fuel available by half all fuel used from one side so boat lists.
Close the balance line suck from 1 side and return to the other Big Problem.
empties one tank much faster than you think while returning to the other tank and overflowing overboard.
We had this situation with a large motor sailor at work.
very light winds so he fueled tanks to Full in Dartmouth, then set off motoring to Pool, supply from stbd, return to Port tank. balance line had been closed.
Boat engine stopped at St albans race in very bad seas, lifeboat called and towed to Swanage, a long slow tow in the dark.
2 days later a fishing boat towed boat back to PYC marina.[