Are you sure the two barrels are connected? I use two water barrels like many others do but only one is connected and the other is prepared ready just in case we have rain or suddenly run out and need a new barrel connecting immediately.
I made my own last year and if we're not on a serviced pitch I find it's great. Fill both aqua rolls and the on board tank and I only have to worry about the waste for at least a couple of days.
No running out mid shower, or in the dark, or when it's raining. Just top up when I get the chance.
All made from bits I had lying around apart from the aqua roll caps which I got from e-bay for a couple of quid. Could have used the originals but I wanted the option to return to normal use if I wanted.
I'm trying to find out how to post photos at the moment, that'll explain things better than I can in words.
I've got both of mine on stands but all will become clear shortly I hope.
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