Should I run two gas water heaters if I don't need to?
If you haven't noticed a lack of hot water, I would say there's no reason to have both water heaters running.  Just make sure that the gas line to the one that's not working is turned off.  While the second water heater is currently doing more work than it would typically do if they were working in tandem, it's not using nearly as much gas as you would if both water heaters were running.