This was a feature of X-Plane 10 but was removed in X-Plane 11. There's no longer a particularly good reason to run the IOS on a second machine. (Unless you're out of monitor outputs on your primary machine!)
Previously, this was desirable because the IOS (and the X-Plane 10 map in general) was quite slow, so the performance hit from running the IOS on your main machine was consider.able In X-Plane 11, though, the performance hit should be minimal, so it's much cheaper (and easier, and more reliable) to just run it off the main machine.
Thus, in X-Plane 11, we expect people to run X-Plane in full screen on their main monitor, and (using the Settings > Graphics configuration) set a *secondary* monitor's usage to IOS.
With that said, if you really want to run the IOS on a separate physical machine, you should be able to do so by setting your second machine to act as an external visual, then going to Settings > Graphics on that second machine and settings its monitor usage to IOS.