OK - here's my solution
Version I: Bigger - special shape fuel tank - then you can fill half of it...
You can make it by welding two separate tanks with few holes in bottom of upper tank/top of bottom tank
Version II:
Two separate tanks connected with fuel lines (system of communicating vessels). When you fit cut-off valves you can close connection and use only one tank. You need only one filler pipe and output - from smaller tank... and if you put fuel level sensor in smaller tank it will show correctly even with bigger tank "open"
EDIT:
v2 attachment include two-in-one Version II