Need a bit of advice …
they gave me the keys to a different truck for next week its a 13 plate daf 105 but its got a huge tank maybe 800L or more on the passenger side and a normal 500L on drivers side ,just wondering how you swap from using one tank to the other ?
They might be plumbed in to work together, after a good few miles have a look in each one to see the levels.
Or have a scout round underneath, you might find a tap of some sort to switch the spare one on/off.
Sure ones not a hydraulic tank just checking 
twin tanks are normal on eu spec dafs 1350 ltrs drain together supposed to full bigger tank first in reality it dont matter
Why do you need to fill the bigger one first if they are both linked, fill them both from the same side but just allow a bit of time for the fuel to gravitate level …
Our old dafs used to drain together, not sure if filling one would fill the other because we had our own pumps and used to back in between them and use one on each side.
Provided the tanks are the standard fit ones then they should drain together. I don’t know for certain on the filling the big one first thing but I would be guessing that the engine draws from the big tank and the transfer pipe between the two is one way, so fuel from the big tank won’t flow into the little one, whereas fuel from the little one will drain into the big one and it will take longer to fuel up.
raymundo:
Why do you need to fill the bigger one first if they are both linked, fill them both from the same side but just allow a bit of time for the fuel to gravitate level …
Most pumps will fill a tank quicker than it flows from one tank to the other. So, if you fill the big tank first, (with cap off smaller tank to allow air out) the smaller tank will be almost full when larger full, so no need to fiddle about filling both. Do smaller first, and you`ll definitely need to do both. Only saves a little trouble but I thought worth it.
Edit. On my factory fitted one there were no valves or taps between the two sides. Beware of filling or opening tops if on a very steep camber: it can flow from one tank straight into and OUT of the other lower one!