Hopefully this is not too off topic, but I had a question as to plumbing for multiple water heaters.

I have 2x 40g 5 year old Bradford White heaters plumbed in parallel. One of the heaters is leaking from metal on metal corrosion (hot water side). The repair guy thinks it may also be cracked from evidence of rust at the bottom of the pan. The pilot stays lit, but after about two 10 minute showers the hot water stops.

Aside from all the problems with the tank and leaking plumbing, what is the opinion on the plumbing? Should the tanks be plumbed parallel or series? I understand the argument for both, but which usually work the best?