A communication device for handling dual cellular system aggregation includes a storage device for storing instructions and a processing circuit coupled to the storage device. The communication device connects to a network via a first radio access technology (RAT); transmitting a “first RAT-second RAT” joint aggregation capability and at least one of a first RAT sole aggregation capability and a second RAT sole aggregation capability to the network; receiving a second RAT configuration configuring a “first RAT-second RAT” joint aggregation; connecting to the network via a second RAT according to the second RAT configuration; and receiving a first data via the first RAT and a second data via the second RAT, after connecting to the network via both of the first RAT and the second RAT.