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What is the correct way to series the cold and hot water in and out of two 50 gallon electric hot water heaters?
Cold supply to tank one cold, hot from tank one to cold tank two, hot from tank two to out.
Is there a reason you want to series the tanks together instead of letting them fend for themselves? In series, you'd be preheating the water in tank one before feeding tank two. If you paralleled them into the system, they'd work independently of each other and still double your storage capacity.