You can only have 1 USB upstream cable connected at a time. You could try a USB hub and see if it allows you to "split" the USB upstream to 2 different sources.
MacBook Pro (13", 2019 Two Thunderbolt 3 ports) connected to the external hub (Ugreen CM314) via Thunderbolt port. Alienware Monitor AW2721D connected to the external hub (Ugreen CM314) via HDMI port. Mouse and Keyboard USB connected to Alienware AW2721D behind port. Alienware AW2721D Firmware: MOB103 MacBook OS: 12.6 I can't control the MacBook via the USB mouse and keyboard connected to the AW2721D? Can you help me out?
AW2721D, MacBook Pro, no USB device control