Complex sentence: Vuk Stefanović Karadžić had found no historical documents on Petar Mrkonjić, while folklorist Stjepan Grčić did not believe Petar Mrkonjić was a historical figure.
Split into simpler sentences: Vuk Stefanović Karadžić had found no historical documents on Petar Mrkonjić. Folklorist Stjepan Grčić did not believe Petar Mrkonjić was a historical figure.