A picture would help. Assuming you have an auto changover regulator.
Close both tank valves and disconnect from the regulator.
Verify both tanks are full, or close to it. I go by weight.
Hookup both to the 2 sides of the regulator.
Slowly open the tank the lever points to, this insures the OPD valve does not trigger. The green/red indicator on the regulator should be green.
Try lighting the stove, it may take a few seconds till the gas gets there.
If it lights you can shut off the stove, close the gas bottle valve.
Change the lever to the other tank and slowly open that valve. Green/red indicator should be green.
Try the stove again.
If it lights both tanks are okay as are both pigtails and most of the regulator.
To verify the auto changeover function:
Leave the lever pointing to the tank you just used and leave that bottle open. Indicator should be green. Slowly open the other valve.
Light the stove again.
With the stove lit, close the gas bottle valve on the bottle that the lever is pointing to. Give it a minute. The indicator should go red and the stove should stay lit. if so the valve changed to the backup bottle and all is good.
If this were a normal day and you noticed the red indicator, you would know the gas bottle it points to is empty. move the lever to the backup bottle at which point the indicator goes green again, disconnect the empty one and get it filled.
With an auto changeover regulator you leave both tanks open. When the pointed to bottle goes empty the indicator changes to red. At that point you change the lever and can close and disconnect the one that is now empty. After refilling, reconnect it to the regulator and open it's valve. It is the standby till the other one goes empty.
The indicator tells you the status of only the bottle the lever is pointing to.
There are many YouTube video's that explain how the auto regulator works.
