The power meters on the Stages SB20 must communicate with each other in order to provide proper data to the StagesBike and connected apps.
Some users want to disconnect one leg and pedal only with the other for single-leg drills, which can cause strange behavior. The power meters use ANT+ to communicate with each other, which allows for limited communication between the two units. As a result, they make some assumptions when one side does not receive data from the other side.
For example, if the left power meter is out of battery, the right meter waits 45 seconds of not hearing anything from the left meter and then assumes the left meter is dead, and it begins to function as a right-only meter, doubling right power. The rule here is: 15 seconds before you get left only power, 45 seconds for right.
Given this, the best way to do 1 legged drills is to leave the other leg attached to the pedal, but only concentrate on pushing with one leg. This will prevent the power meters from assuming its partner power meter has run out of battery.
The second best option is to pedal lightly for 15 or 45 seconds (depending on right or left-side drill) with one leg, then begin your drill.