Wanting to install 2 gas hot water heater should I install them in series or parallel.
Will not have different water zones.

First question... why? Two are less efficient than one larger one.

Is it because you need a huge amount of water? Then look into tankless versions.

should I install them in series or parallel
That depends upon on what you want. More volume or one heater to preheat the other. What's your pleasure? Back to you, Tom

I have the same question as smooothy, go for a bigger or the tankless version, for more info about series or parallel clickon the link below, good luck.

John

http://www.dallascityhall.com/pdf/Building/ParallelSeriesPiping.pdf