Do you have a boiler or a hot air furnace? Usually, if you are using a boiler, you'd install an indirect water heater rather than a separately fueled WH. A coil in a furnace could be for space heating, but that normally isn't run off of a WH. A clarification might help here.
Now, whether you need both WH is totally user dependent. Were they both put in at the same time? OR, did you start with a 30g, and then find out it was too small, then add a 50g to it? There's less standby loss with a single tank. Recirculation is a convenience issue that helps you from wasting water waiting for the hot to get there. Unless you use two WH, and place them each closer to where the hot water is necessary, having two usually isn't a benefit, other than the increased volume available.
Depending on how you use water, your 'need' might be met with a smaller tank. If three people are all showering at the same time, what you have in total volume might be necessary. There are ways to minimize this by installing a waste water heat recovery system (if you have the vertical room for one). This will not work if you use the hot for say a tub, since you need all the hot to fill the tub, and there's nothing going down the drain WHILE you're using it to help offset the energy going in.
At 20-years, you're well over the norm for needing a replacement.