This is the expected behaviour. I would suggest use the “view only” option for users who shouldn’t use events in that specific moment. Then you can change dynamically from view only to allowing control.
When user 1 on client 1 presses Alt on one machine and then another User 2 from client 2 scrolls, the result is that the picture zooms. This is because in version 8 NoMachine separates (and “arbitrates”) remote events from local events, but all remote events are considered coming from the same device. In version 9 we will add separation (precedence) between remote events coming from different connections.