Outbound calls don’t relate to the queues system at all!
Are you saying you want the queue system to direct incoming calls to the agent’s device whilst the agent’s device is in a RINGING state on an outbound call, and the agent’s device is a SIP device, or similar, that can physically support the tow calls?
If so, ringinuse may help, but it is up to your device to reject incoming calls once the outbound call is in the UP state.
I suppose you might be able to use a local channel between queue and the device and have it check the device state, and examine the device state on the actual agent device.
This assumes the agent’s device is local to your system, so there is a valid device state for that device, rather than for a trunk.