Parallel Water Heaters, One Doesn't Run
Piping 2 water heaters in parallel is much more than just piping, it is an art. If either tank has longer sections of pipe for the in and out connections, before the common piping, then the one with the shortest piping will get most of the flow. 

The preferred method would be to start with 2 tanks from the same manufacturer, the same size and the same model number so both are exactly alike. You may need to contact a "good" plumber and ask him to do this for you. Myself, I prefer to pipe 2 tanks in series, it is easier to do, yields the same results, and is fool proof. 

One last idea, if you need this much hot water to fill a large Jetted tub, you could use a single 50 gallon water heater and for the tub add a dedicated electric "point of use" water heater that would only be used when the water heater begins to run out of hot water. This is what I did.