Diagnosis
PBP mode has specific active signal requirements. It will not activate if one input is idle or in sleep mode.
How to Fix It
Connect the two source devices to two different inputs on the monitor, e.g., USB-C for laptop 1 and HDMI for laptop 2. Ensure both computers are awake and outputting a signal. Press the joystick, navigate to Picture > Screen Split, and select PBP. Choose the two input sources to display.
If That Didn't Work
The monitor may be defaulting to a resolution/refresh rate that is incompatible in split mode. On the source device, manually set its output resolution to 1720 x 1440 (half of 3440 x 1440) at 60Hz before enabling PBP.
Heads Up
In PBP mode, advanced features like Adaptive-Sync, HDR, and specific color gamuts are disabled. The USB-C downstream ports and audio-out will only be associated with the input selected as Source 1 in the PBP menu.