Can we connect external Bluetooth Dongle in Android Tablet?
If you are able to connect an external Bluetooth Dongle to the Android device, it ought to work for you, even with an intrnal Bluetooth present.   

Android devices with OS v3.1 and above shall support USB host capabilities. So, it is advisable to experiment with devices with the above operating systems. However, it will be on your end to make the dongle work for you; i.e. by creating a separate application for the same.