FWIW: I have two 40 gal tanks plumbed in parallel. I only need one. I alternate the tanks every 6 months (shut off the intake and turn the gas down to pilot) and after over 3 years no problems. Zero odor. During 2011 I was away for an entire year during a military deployment and left both tanks full and the pilots lit. I had someone run some water in the house every 3 months or so. The advice I got was to leave the tanks either full or BONE DRY. It was easier for me to leave them full and the pilots lit to prevent condensation. When I got home after a year no problems, no odor. I understand this might not work for everyone but so far it has worked for me and at least for my needs it seems to be the best solution. Might be worth a try……..
As far as the tanks becoming unbalanced: The plumbing to the two I have is exactly the same with equal lengths of pipe to each after they divide. I do not notice any change when I switch from one to another. Is the unbalancing something one can detect or is it something that only has an effect when the two tanks both have water flowing through them??