some trucks (frieghtliners et al) with two tanks only show on the gauge what is in one tank, there is a crossover line that connects the two tanks so theoretically the levels will be the same, assuming you are on relatively flat ground...
the truck above is a Peterbilt model 379 and every one of them that I've driven has had two tanks, one fuel gauge, and a three-position switch on the dash to show either the left, right, or avg of the two...on this truck there is something wrong somewhere, the switch or idk, but only shows me the right side tank...however since I have the ability to physically remove the fuel caps and peer inside with a flashlight it is ultimately my fault it happened...
I've driven a couple of different Volvos and they have a visual and audible alarm when you get below a quarter tank...that makes a lot of sense to me...