If I understand what I read, you are correct. If plumbed in series, the cold from street/supply should connect to the cold inlet on first heater. The Hot/discharge line out from the first heater then is connected to the cold/inlet of the second heater. The second heater Hot/discharge then is connected to the hot water distribution lines in the house.
If taking water out through the inlet, then you draw out the coldest water while the hottest water remains at the top of the tank (heated water rises)
One quick test I do is to simply feel the cold and hot lines after drawing down a little hot water, just to verify the correct connection and thus correct location of the stop valve on the cold water inlet.
Jim

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002