ahhhhh  one is empty and one is full?

When I had my 1989 F150 with dual tanks, I could never depend on the gauges.
When you change the gas tanks.......and you will, you may need to replace
the fuel pump/gauge float in the tanks too.  In my case the front tank
leaked first.  4 or 5 weeks after changing the front tank, the rear tank
started leaking.  The rust on the whole tank was real bad.

We cleaned up the front fuel pump assembly as best we could with a small
sandblaster and reused.  Replaced rear fuel pump assembly with some sort of
rebuild, and if I recall correctly; neither worked the greatest.  There must
have been a bad common ground wire somewhere that needed to be cleaned up
and got over looked.

I would set the trip odometer at zero on a full tank and top off at 200
miles. It was just a method that I used on my 86 F150 and 86 Bronco for many
years. Even from the factory my 86 F150 fuel gauge was unreliable.

So, to answer your question, There must  be a bad common ground wire
somewhere.