Remove the sensors from the harness connections. If the senders are two wire type, check for voltage at one terminal with key on engine off. If voltage is present, check for gauge movement, then briefly loop the two terminals together while checking for gauge reaction.
If the senders are single wire type, remove connector. Check for gauge movement with a completely open circuit (key on, engine off), then check while briefly connecting the harmess connector to a good clean ground.
If the gauge reacts to the tests, your gauge and circuit are good, and the sender is suspect.
I agree the simultaneous loss of two gauges points to a power supply problem, or incompete ground.