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You can't simultaneously register the ABC and XYZ dissector for port 3100, as there would be no way to determine whether it's protocol ABC or protocol XYZ. You can, however, register the ABC dissector for port 3100 and the XYZ dissector for ports 1024 through 3099 and 3101 through 9100:

dissector_add_uint("tcp.port", "3100", ABC_handle);
dissector_add_uint_range_with_preference("tcp.port", "1024-3099,3101-9000", XYZ_handle);