Dual Watch mode, described in the manual on pages 18 and 19, permits you to monitor two channels "at once".
Use F+9 to enter this mode and the same F+9 to exit this mode.
Look for "DW" on the display.
NOTE - The rest of the keyboard is locked-out while in the DW mode. It'll just "beep beep" at you. if you press anything other than F+9.
Also, see this post about keypad lock feature: