When connecting to an agent with multiple monitors using a Remote Control > Private session, the display selector drop-down is not available.
Private sessions enable administrators to connect to a machine, logon and remote control the machine without accessing the console. An end user working on the same machine at the same time cannot see the administrator's private session.
This is achieved by connecting to a "virtual" desktop session. There is no interaction with the physical display(s) that are attached to the machine.
Kaseya Remote Control - v7.0 and later