You can drain the sediment from the heaters one at a time in the two normal fashions.
1. (starting with the first heater) Simply open the drain valve and let the water flow until it is clear.
2. (starting with the second heater) Turn off both heaters' heating (turn the knob to pilot). Turn off the cold water supply to the first heater. Open a hot faucet upstairs. Open the drain valve and let the water flow until it is clear. Turn on the water and have the hot faucet gushing before turning the heat back on.
In both cases no one should be using hot water for several hours before you start the procedure. If you choose to do both procedures at about the same time, wait several hours in between. But you can do both tanks in quick succession for one procedure. Typically draining the sediment every 6 months (one procedure, alternated) should be enough each time.