In the ASDA RDC where I go sometimes, some of the older Netto Scania’s have 2 tanks on them (both hold about 250 litres between them so I don’t know what the point of having 2 tanks is haha), anyway…the O/S tank doesn’t ever drain any diesel and you never use more than 50 litres in a shift anyway so it’s ok to run the truck, but my question is…would the truck just start to use O/S tanks diesel if the nearside tank emptied ?
I’ve got the big 380 ltr square tanks on mine. If the nearside tank is filled it will not balance over, if the offside tank is filled then it will balance over. Fill them both together (in any order) and they both go down together.
On another note, why are scania fuel guages always so inaccurate. I have had mine on the empty line and there has still been over 100 litres in it! Its not just this one, its every scania Ive had thats been inaccurate.
Happened to me last week, did like a few have suggested and put airline in with rag round it. Just a couple of good bursts and it cleared it. Why anyone would want to put 100psi in is beyond me. I also gave the tap a little tap incase there was a bit of crap in it. I found it didn’t move straight away but after half hour you could tell by fuel gauge it had started to balance
The fuel guage isn’t inaccurate Skids, it’s just a gaffer’s device to stop muppets running out of Diesel. 
The only problem you have then is once you know there’s still 100 litres in it at the empty mark, you still just try & get to the next one. I’ve found pyschologicaly it does generally make you fuel up sooner though.
If you then jump in another make of truck, DAF say with an accurate guage and think they must give a bit of lee way like Scania, you may end up with egg on your face. 