Two 100 gallon propane tanks but only 1 gauge?

sounds like they only put 75% in one take . the other take valve might have been closed . If you line ties in to both tanks in a tee. They should show the same level in both tanks as the gas burns off . It should not drop in one tank and not the other . or maybe the valve is closed on the second tank and the gas was not burned off and is still full but the valve is closed. check your tank valves. either way something is going on the gas should not be used out of one tank and not the other .