What you're seeing is pretty normal on many modern fuel level circuits. The fuel gauge module or PCM doesn't always feed the sender with full battery voltage. Instead, it often uses a regulated reference voltage and measures the return signal through the sender's variable resistance.
With the sender unplugged, a digital multimeter can show something like 5–6 volts on both wires because the circuit is essentially open and you're measuring through the gauge electronics. The meter's high impedance doesn't load the circuit, so you can get readings that seem odd at first glance.
Checking the resistance of the sending units themselves is a good next step. If both senders test within spec and sweep smoothly through their range without dead spots, then I'd start looking at wiring, connectors, or the fuel level processing module rather than the senders themselves.
The 5.5-volt reading by itself wouldn't immediately concern me; it's often just a byproduct of how the fuel level monitoring circuit is designed.
