We would like to travel to Croatia and Slovenia in late September/early October 2020 and will be flying from DFW. I've only done round trip flights to and from Europe before, but the geography in that region makes flying into one airport and out of another, say Dubrovnik and Ljubljana or vice versa, seem to be the most logical choice. I'd welcome any suggestions on booking that kind of itinerary - in a search of this forum, I see Frankfurt suggested - any other ideas? Thanks so much!
I'd simply go to a website like Google Flights, choose Multi-city with DFW as the origin of the first trip and the destination of the second. Don't worry about intermediate transfer points like Frankfurt; choose places you might reasonably want as your final destination of the first flight and the origin of the second. Dubrovnik and Ljubljana are good ones to try. If those result don't look good, you might try Zagreb or Venice instead of Ljubljana.
I can’t comment on US flights, but bear in mind that many places, especially on the islands close at the end of September and start to wind down earlier than that, so although I don’t know your itinerary, you may wish to bring it forward slightly.
Since you're flying from DFW where American Airlines is headquartered, look at their direct seasonal flights between Philadelphia and Dubrovnik (so just one connection). They recently announced that the seasonal Dubrovnik flights will continue until late October in 2020. If you can't find a decently priced flight to or from Ljubljana, consider flying into Venice, Munich or Zagreb and taking a train or Flixbus between there and Ljubljana. Flying to Ljubljana from Frankfurt or Munich might also be a possibility as Lufthansa has recently increased flights to Ljubljana after the September 30 bankruptcy of Adria Airlines.
And yes, Dubrovnik and Ljubljana are quite some distance apart, so flying into one and out of the other makes good sense. At that time of year I would start in the north and travel south, but either direction works.
Edit: Note that there are 2 other recents postings about flying to Ljubljana, one in the Croatia section and the other in the Rick Steves Tours section. You may find some useful information in those postings too.
I have found that flights from the US to Coatia/Slovenia quite expensive compared to other places in Europe. And they all seemed to have a couple of connections making for very long travel days. So what I've done is flown into somewhere easy/cheap(er) to get to - like Venice and then go on to Slovenia/Croatia. This means you really need to be willing to spend an extra night or two in Italy (or Germany or France or where ever) - booking two tickets on separate airlines is the same day is too risky if the first flight is delayed. But time is not an issue then you can have a nice trip with a couple days in one or two other countries book ending your trip to Croatia/Slovenia. EasyJet flies routes from Ljubljana and Dubrovnik to many other cities in Europe, although less so when you get into October.
For example, my most recent trip we flew into Italy, took a ferry from Venice to Rovinj, Croatia. Then from Split we flew EasyJet to Lyon France. A previous trip we flew EasyJet Rome to Dubrovnik and Split back to Rome. And yet another trip I flew EasyJet London to Ljubljana and took a train/bus combo from Slovenia to Venice. All kind of great combos that cut down on very long travel days and expensive flights - but you need to be willing (even want) to have those few extra days in other cities to make it work.