We're talking about a 70s beater pickup truck modified as only a 80s/90s teenager has spare time to modify, so I have no idea if this was normal for construction grade trucks or not.
The theory is he could operate his gas powered generator/welder or gas powered air compressor off one tank at a job site, and even if he ran it dry he could drive home on the other tank. There was also some manner of fuel transfer pump(s).
And then you forget to fill both and you are stranded... :-D
When it ran out of gas, you had to switch a foot pedal to the emergency tank, then you had maybe 10km to get to a Petrol Station before you were stranded.
Filling up both tanks kind of sucked, back when gas was $3.50-4.00/gallon.
We should just go back to the 70s-era centre-mounted fillers behind the license plate holder. Those were great, until you got rear ended or it was frozen solid.