There is nothing unique about a Reliance water heater. They hook up the same as all other storage water heaters. Cold water into the right side, hot water out the left side, electric to the “J” box on top, or gas to the control on the bottom. Why do you think yours is connected wrong?
If it is not hooked up, you will want to have a plumber connect it so he can be sure that the ventilation, flue, and gas is large enough for two heaters, and that they are connected to give the maximum hot water to the apartments. If it is hooked up and not giving enough hot water, you need him to do the sames things to see if it is working properly.