The right encoder on the front panel of a Red 4Pre, 8Pre, 8Line or 16Line can be used to control multiple outputs at once.

To select which output pairs you'd like to control using the front panel dial configure the "Monitoring" tab in RedNet Control:

The pairs of channels that are selected will show up in blue in the "I/O Control & Metering" to show that they're being controlled from the front panel:

Note that the main Monitor L/R outputs are always controlled by the encoder on the front panel (highlighted in orange in the above image).