I have a natural gas water heater and connecting an electric heater to add for more hot water demand.

We really don't know what your question is. Can it be done? Yes? Do you have the electrical capacity? If you are sure this is the way to go, you can repipe it in series or parallel. In series, the hot water out of the first heater goes into the cold water inlet of the next heater. In parallel, you need 2 sets of pipes with 2 cold feeds and 2 hots out connected to a common pipe. However, you don't have to balance a series system and you may have to put flow limiting devices on parallel to prevent back flow. Most go series. Have you just considered a better larger heater? Mine is 70 GAL.