Fuel Gauge Inop

The most common cause of condition you describe is fuel inside the float. 2 senders doing same thing is probably 2 different floats so thats not the problem.
Here's additional thought... to pull these senders requires tank be drained below half tank. Many owners who install the aftermarkets sender do get a full reading on their gauge when they fill it up. Later down the road when the gauge reads near empty, only 7 to 9 gallons are required to fill it. So... they install a meter match unit to make their gauge read correctly with the incorrect sender. So I guess my question is have you filled the tank?
I didn't see year model but I'll bet its a late model with the 13 ohm fuel gauge. The earlier 20 ohm fuel gauges seem to work better with aftermarket senders.