As Dave mentions, you could have driver A initialize two virtual interfaces. one assigned to the instance of InterfaceA and the second virtual interface assigned to the instance of InterfaceB. You could also create a modport to restrict the access of interfaceB signals to readonly. This should give driverA full visibility of signals in interfaceB.
The drawbacks of this solution, is that the driverA becomes dependent on interfaceB. You could architect driverA to run in two modes. one where interface B is present and the second where interfaceB is not
Another option is to try using callbacks when the monitorB senses a change in the desired signal. This callback function can be triggered to drive some signal in interfaceA/
Accelver Systems Inc