I have an iMac and a PC with a Dell external monitor.
I currently have the mac setup to use the dual monitor, but also want to be able to share the Dell with PC when I need it without having to make modifications to the mac.
if u are using 2 different keyboards and mice then u can use dual input option on your dell LCD and just switch. or get KVM. another option is to use remote desktop client on your mac to connect to your PC via RDP and run your PC headless