@skynet OK, now I get the confusion

Confusion entirely caused by me, sorry! - Kas1e originally asked for the best way to remote desktop an Amiga machine, and yes, that would be like the screen share feature in Mac. For that you need things like VNC. I don't know the answers here as I don't do it. I believe I tried it, but it wasn't quick enough for me and I didn't like it.
I went a bit off topic and asked about sharing the keyboard / mouse between two systems which is something different really. That is where we have two physically unique monitors, presumably sat next to each other. One the Mac / Pc, the other the X5000 and just one keyboard and mouse. When you move your mouse from, say, the edge of the Mac screen onto the X5000 screen, the control seamlessly goes with it. This is more like continuity, when you have an iPad next to a laptop for example. For this we have MKShare client / server, or Barrier / Synergy client / server, and probably others too. IMO, this works well.