Draining 2 series connected water heaters?
Since you didn't say it, and it matters - first, shut off the power or fuel to both heaters, so they are not dry-fired, which will quickly destroy them. This is very important any time a water heater is not full of water.
To flush the second tank, if they are connected in series, consider the first tank as if it was a pipe.
So, shut off supply, open hot taps, drain both, turn on the supply to flush the first, close the drain on the first, let the first fill until it runs into the second, flush the second, close the drain on the second, fill until no more bubbles come out the hot taps, and only then turn the power or fuel back on.
Might want to check the anode rods while you are there.
Be alert to the possibility that malfunctioning thermostatic mixing valves (as on a shower) could route cold water back to the hot side.