We have been to both Croatia and Greece.
You might look for a cruise that visits both countries. If you plan to visit any Greek Islands, then a cruise is great for that. Also, many cruises visit Split and Dubrovnik.
If you want to visit other places in Croatia than Split and Dubrovnik, you could do that after the cruise.
If you don't want to take a cruise, visiting Greek Islands will require taking ferries, which can take up time. Flying in to these islands is more expensive.
I recommend at least three days in Athens. The Parthenon is amazing. Also, see the New Acropolis Museum and the Archaeological Museum. Consider a trip to Sounion to see the Temple of Poseidon as well as Delphi and Corinth and the Corinth Canal.
For the islands, you can do the three island cruise from Athens that visits three islands close by. Also, Santorini, Mykonos, Corfu and Crete are great.
You could do an open jaw ticket, flying into Athens and out of Split or a nearby airport, then find a one-way flight from Athens to Split.