Multiple Computers - Multiple Monitors - One Set of Inputs

@Tate  The Dell displays with KVMs that can handle additional displays would handle the additional displays via DisplayPort MST, specifically daisy chaining. The problem is that macOS does still does not support that. It’s been the subject of confusion and complaints here, on Apple’s forums, and elsewhere for years. So you’ll need an external KVM. If you’re dealing with newer systems and at least one 4K display, I’d suggest the IOGear GCS1942. It can handle dual displays up to 4K 60 Hz each. With recent Mac Minis, you’d use a pair of USB-C to DisplayPort cables to connect each Mac Mini to a bank of the KVM. A friend of mine has that KVM for a similar setup and has had a very positive experience with it.

@Tate  One other thing I’ll mention. Mac systems DO support Thunderbolt daisy chaining. So if you find a pair of actual Thunderbolt displays — not just USB-C displays — and at least one of them has a Thunderbolt output for daisy chaining, then you could go with that solution. But that would narrow down your display options pretty drastically, almost certainly ruling out the old, dedicated display you already have.

@Tate  I'm sorry, I misread your original request.  So you want to have THREE total displays involved, with one display shared and then separate dedicated displays for EACH of your respective source systems?  In that case yes a Dell display with an integrated KVM will serve your purposes.  If you plan to use that shared display such that you'll only allocate the entire display area to one source system or the other at any given time, then that's easy.  If on the other hand you plan to use it as a shared display via PBP such that you can view content from your two systems simultaneously, then you may want to consider an ultra-wide (21:9 aspect ratio) or even a "super-ultra-wide" (32:9 aspect ratio).  Examples of the latter would include a 49" 5120x1440 display, which is essentially two 27" 2560x1440 displays fused together, and therefore when used in PBP mode, each system will see their own 1440p display.  Note however that in PBP mode, typically the keyboard and mouse will not auto-switch systems when you try to move the mouse cursor "across the edge" to the other system, as Logitech Flow and other similar solutions can.  Normally you have to manually tell the display to its active USB upstream interface to the port for the other system.  Some Dell displays may support switching USB upstream interfaces automatically, but even if that's available, it might require Dell Display Manager to be installed.  I remember reading that Dell intended to release a Mac version of that, but I don't know if that actually happened.