It would be great if you could isolate it to one tank, by disconnecting the second one. It could be a step toward figuring it out. Is there any way to see the water in the tank? You could just isolate them and have a look after a big rain storm (conceivably). I don't know if you could try to run it on only one if you wanted. Probably.
I can easily isolate 1 tank at the T piece which is a copper/brass compression gas T.
There is no way of knowing if there is any water in the tank until you drain it, then in a see-through container, you can clearly see the two liquids with a line where they meet.
