Jim:
It's actually a good idea to have two water heaters plumbed in series. That way, if and when one water heater kicks the bucket, you can simply switch over to the other water heater while the first one is being replaced.

Also, you can use that second water heater while doing maintenance on the first heater, like replacing the anode in it. That way you're not in a situation where you go without hot water unless and until you can replace the anode.