AHC/WI : Permanent Partition of Francia into Neustria and Austrasia .

The Franks had an established tradition of splitting up their realms between the sons with the Kings seen as ruling the same realm . However these failed to produce a scenario similar to that of the HRE and West Francia . In your Opinion , what would be best way to keep the respective realms of Neustria and Austrasia ( and Maybe Burgundy as well) from reuniting with minimum a PoD of 613 and maximum of 756