They were actually more divergent in this period than they are now. Modern Serbian and Croatian are both based off the East Herzegovinian standard of the West Shtokavian dialect of Serbo-Croatian - a result of standardisation in the 19th and 20th centuries, pan-nationalism, and Ottoman advances causing mass migrations of speakers of the various South Slavic dialects. In the 1300s, Croatian was based on the Chakavian dialect, now spoken near-exclusively on Adriatic islands, and Serbian was based on East Shtokavian, which is slightly closer to e.g. Bulgarian than modern East Herzegovinian is.1). If that's your concern, then I think it would be more appropriate for South Slavic to be renamed to Serbo-Croatian. They may all say they're different languages, but just like Arabic, they're the same language.
Also of note is that the Zagorje and Drava regions (the historical Kingdom of Slavonia, around Zagreb) spoke the Kajkavian dialect, closer to Slovenian than to the Croatian Chakavian or the Serbian East Shtokavian. With the granularity we see in other cultures (splitting Romanian into Wallachian, Transylvanian, and Moldavian!) I think this could merit the addition of a Kajkavian-speaking Slavonian culture that may later merge into the Croatian, though no doubt this would upset a lot of people!

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