Would the two nations be merged in relative peace for most of modern times, or would they find a way to be rivals? Would Bogomil heresy or later Islam or Protestantism ever prospered in the region?
PoD - a) Croats take Christianity from Constantinople, write in Cyrillic script, and never gravitate toward Rome or b) Serbs take Christianity from Rome, write in Latin script and never gravitate towards Constanople.
How would either situation affect dynamics with the ERE, Bulgaria, and possibly later the Latin Empire or Turkish invaders from Anatolia and Thrace?