Serbia actually DID oscillate between Orthodoxy and Catholicism for a while, though not to the same extent as Bosnia, so a Catholic Serbia isn't out of the question.
However, it would not result in either a "Greater Croatia" or a "Greater Serbia". The two states would probably still remain separate for a while; the difference in religion was enough to foster different national identities, even in the middle ages [1], plus there was the whole matter of there being different royal houses in Croatia, Serbia, Bosnia, and Zeta/Duklja (early Montenegro), which as we all know is a sure fire way to encourage regionalism.
With a shared religion, though, I do see them uniting much, much earlier, and under potentially very different circumstances due to the butterflies. The unification may be similar to the OTL unifications of Germany and Italy, and just as those countries didn't take the names "Prussia" or "Piedmont" after the states that masterminded them, so would this state not be called "Serbia" or "Croatia".
Slavonia, Slovenia, Slavenia, Sclavonia, Sclavinia, etc... Something along those lines is a much more likely name for a united South-Slav state with a single religion. Hopefully by the present day the idea of anyone identifying as "Serb" or "Croat" would be unthinkable, they would all just be "Slaveni", just as they were originally.
[1] This is in large part because of why the terms "Serb" and "Croat" exist at all. Originally, the South Slavs were just a single group, the Slaveni/Slovani (a name preserved in the OTL name "Slovenia"). Then, between the 7th and 9th centuries, other tribes, both Slavic and not, started moving into the area. The Croats were originally either a Slavic or Iranic tribe (personally I lean more to the Iranic side, but that's neither here nor there) who asserted themselves as rulers in the western part of the Slaveni lands (and, if they were and Iranic group, quickly assimilated by the local Slavs, in a manner similar to the Bulgarians), where proximity to Italy made them lean toward Catholicism. The Serbs originated as a Slavic tribe that initially established themselves in what is now eastern Germany (some still remain there, though their language is now more similar to Polish/Czech and is called "Sorbian" to avoid confusion). They were invited to establish themselves the Balkans by a Byzantine emperor for the purposes of acting as allies in the region, which was consistently beginning to cause problems for the Byzantines. The Serbs established themselves as rulers over the eastern part of Slaveni lands, and having been invited by the Byzantine emperor naturally leaned toward Orthodoxy. That initial tribal difference, as I see it, was as important to the establishment of differing Croat and Serb nationalities over a Slaveni ethnic base as the religious aspect.