Combining Greece with Croatia? (3 weeks)

I would spend the whole time in Greece this trip. We have been to both, Greece two times. Both are beautiful but prefer Greek culture, food.
Have you explored how to get to one from the other?

@Suki Thanks for your response. No not yet, perhaps take a flight from Athens (if we start with Greece) to Zagreb, travel down to Dubvronik and fly back to SA.

I typically take 10-day trips because that's the ideal amount of vacation for me to take at once, so I know you can have two great trips of that length, but I tend to favor going to one country. One benefit of that is that you can balance bustling places (like Dubrovnik) with some time off the tourist trail like a less popular island or an inland national park. That is just a personal preference, so if the scheduling works out for you, go for it. Order will entirely depend on flight logistics, but I would want to start with Greece for maybe slightly less crowds.
I have been to Croatia four times including my honeymoon, so it has a special place in my heart, but having finally just recently gone to Greece, I would be hard pressed to pick. I will say what was extraordinary for us was seeing all of the ancient sites and extraordinary scenery in the Peloponnese (with very few crowds). After seeing the beauty of so many Croatian islands without crowds, visiting Mykonos and Santorini just did not sound appealing. All just impressions of course!
While the seafood in Croatia is wonderful, I am also a huge fan of Greek cuisine.

I also come down on the one-country side. I've been to Greece 3 times (none of the trips being recent), and though all the trips were great, my favorite was the one on which I got down to the Peloponnese Peninsula. With 10 days you'd have just an Athens + islands trip, and I prefer more variety.

Altough I've also made multiple trips to Croatia, I haven't had a comprehensive visit to that country.

If you decide to split your time, I recommend not trying to include three Greek islands. Dealing with flights or ferries will make two enough of a challenge, I suspect. I'd try to fly right to one of the islands initially.

Edited to add: Although the views from the ferries are nice, the time spent hanging around the port (or the airport) waiting for departure isn't exactly prime sightseeing time, so a trip with a lot of short island stays will probably mean you see less rather than more.

Flying between the two countries is the only practical method of transfer. There is no direct train, bus, or ferry.

Yes, travel from Greece to Croatia is not simple. We just did exactly this. We had to go to Budapest. There may be a better way, but we did not find it. Fly Athens to Zagreb.