Larry thanks for your last reply to the corrosion problem. I'm going to have a plumber look at it.
My question is how should the two water heaters be piped.
As you can see from the picture they are currently piped with a sort of parallel piping with the heater on the right being further away. It seems that most are piped in series.
We would also like to be able to shut off one of the heaters for a couple of months a year.
Ben, that piping is a mess! You are correct in thinking that they are piped improperly. The way it is piped now you will be drawing the majority of your water from the left hand tank. Water wants to take the path of least resistance. They should be piped in a reverse return configuration to draw equally from both tanks. I would definitely get a pro out there to fix it. As well, your exhaust piping may kill you and others! It should not be sloping downwards! The melted insulation on the right hand tank tells me that you are getting flue gas spillage into the room. Do you have a carbon Monoxide detector in the room? I don't mean to sound harsh, but this really needs to be corrected. (and yes, if they are valved off you can shut off one heater for a portion of the season, just make sure to turn the gas valve down to the vacation setting)