Do you mean macOS DDPM (Dell Display and Peripheral Manager) or the older macOS DDM (Dell Display Manager) ?

I cannot find any statement that declares our DDPM/DDM works with any non-Dell monitors.

The newer DDPM states =
Which Dell monitors are supported on Dell Display and Peripheral Manager for Mac?

The older DDM states =
Dell Display Manager (DDM) works only with select Dell monitors.

Hiya, Yes the old DDM has upgraded to DDPM.

Are you saying I need 2 Dell Monitors and then it would switch seamlessly between the 2 macs and 2 monitors?

Yes, using DDPM on two Dell macOS validated monitors.

Fab and that doesn't matter what inputs are being used?

So I have the;

Dell UltraSharp 40 Curved WUHD Monitor - U4021QW

and the:

Dell UltraSharp 34 Curved USB-C Hub Monitor - U3421WE, this is on order, it looks like they are in the compatibility list.

So in DDPM, how does this work, and how does it know to switch the both monitors at the same time from one computer to the next, I only ask this as I got confused enough in the KVM settings yesterday, plus it would mean both monitors have the USB hub on them, if that makes a difference.

Also is this the right keyboard and mouse combo to go with them? 

Dell Premier Multi-Device Wireless Keyboard and Mouse - KM7321W - UK (QWERTY)
https://www.dell.com/en-uk/shop/dell-premier-multi-device-wireless-keyboard-and-mouse-km7321w-uk-qwerty/apd/580-ajqo/pc-accessories

Open the online macOS DDPM User's Guide page 36 to read on how to setup the USB KVM for two PC.

The KM7321W sales page states MAC compatible. The KM7321W drivers page does not show any macOS drivers. So I can only assume that the macOS will provide basic drivers?