A mobile communication device including a first RF device, a second RF device, and a controller is provided. The first RF device performs wireless transmission and reception utilizing a first RAT. The second RF device performs wireless transmission and reception utilizing a second RAT. The controller uses a first subscriber identity to make a first call or conduct a first data session via the first RF device, determines whether a dual connectivity on the first RAT and the second RAT is supported in response to a request for using a second subscriber identity to start a second call or a second data session via the second RF device, and allows the second call or the second data session to start during the first call or the first data session in response to the dual connectivity on the first RAT and the second RAT being supported.