Croatia and Greece 2 weeks?

You’ll need at least 10 days for the highlights of Croatia and could easily use two weeks there.

If you want to add Athens, Santorini and Mykonos, you’ll need to add time for an optimal trip.

In April you will not be really enjoying the beach much--seas are absolutely frigid. I would adjust to more of a town and city kind of trip.
I don't recommend these two countries in two weeks either--too much hopscotching and skipping over too much.

This is definitely possible! You could do a week in each. In Croatia, I would decide whether you want to see more of the north or the coast/south/islands and select 2-3 destinations. For Greece in one week I personally would visit Athens and one island. April will be a bit too cold for the beach still so you can take that into account with your planning.

What do you consider the highlights of Croatia? Are you okay with it being too cold for beach?
I see no direct flights from Zagreb or Split to Athens in April (at least according to Google, perhaps there are some seasonal flights). There are direct flights from Dubrovnik to Athens, so you could fly into ZAG and from DBV to ATH, home from there. But I still think you lose too much time in transit for this to be viable.

With "two weeks" I would suggest you pick EITHER Croatia or Greece. One or the other, not both. These should really be two separate trips – and each country deserves a lot more time than you would have in your proposal.

Visiting Greek islands eats up days by the handful just getting around. Croatia and Greece are not very well connected.

Also, you should define exactly what you mean by "two weeks." Is that two weeks from when you lock your front door at home, until you get back home? (That would mean your trip would only have about 11 or 12 full days on the ground in Europe.) Look at exactly (and specifically) how many days and nights you have to spend in Europe, on the ground, not including your arrival or departure days. Do the math, I think you will find you will want to stick to either Croatia (can be done pretty well in 11-12 days) or Greece (surprisingly hard to do in 11-12 days when you toss in a handful of islands), not both.

I am going to be the odd man out here again. Visiting Athens, Mykonos, Santorini and highlights or Croatia in a couple weeks sounds like a great trip. (So does spending two weeks in Greece or two weeks in Croatia!)

Heck, I'd be tempted to add a stop in Slovenia!

Pacing is a personal choice. I like the tempo recommended by Rick Steves-- see his country recommended Itineraries. This forum seems to be dominated by excellent, thoughtful, well-travelled folks who recommend slower speeds. Unless I was on a beach-y vacation-y mood, a day is all I can take of Santorini and Mykonos-- and two days is more than I personally need in Athens.

Whatever you choose, I'm sure you'll have a great time.

Happy travels!

This is just the beginning of the conversation, hopefully. We've provided our initial thoughts, now hopefully the OP will come back to discuss specifics and cobble together a great trip regardless.