You need to remove the CameraViewport offset from the mouse positions.
So if your second camera has it's viewport set to: 200,30,100,100
You should use: CameraPick camera2,MouseX()-200,MouseY()-30
So if your second camera has it's viewport set to: 200,30,100,100
You should use: CameraPick camera2,MouseX()-200,MouseY()-30