I've had the same issue. One strange temporary workaround I found is as follows:
1. On the master machine, toggle on the option to use the side view computer (that won't properly sync with the master machine) as an external view
2. Also toggle on the option to use the same side view computer (that won't properly sync with the master machine) as an IOS, but on that side monitor computer, set the machine type as external view, not IOS.
With this setup, the side view machine will properly sync and follow the master machine view WITH THE FOLLOWING CAVEAT - the weather and date/time settings made on the master machine do not propagate to the slave machine.