Travel requirements between countries?

Slovenia and Croatia are both part of Schengen. The border should be fully open, unless something untoward happens.

There is always a chance of having your passport checked at the border (or within a country) even between Schengen member countries, but no other special requirements, other than a passport, are needed.

As for getting between the two, both bus and train are a very long day (8 hours or more), I would probably look at flying, unless you want to add a stop or two along the way.

Thank you both. That’s what I thought, but my husband (who has done no research or planning) tried to tell me otherwise. lol I appreciate your help!

Your husband may be thinking that Croatia is not part of the Schengen zone. This was true up to January first of 2023, but since then, most all border controls between Slovenia and Croatia have been removed.

While, technically, there should be no border controls between Schengen countries, I suggest you have your passports available just in case. I've run into a some checks in the last few years.

I agree with Frank II. That is our experience also.

Croatia, Slovenia and Italy have enhanced border checks in place which should just add time to border crossings as they check paperwork:

https://schengen.news/croatia-italy-slovenia-agree-to-maintain-internal-schengen-border-controls/

Last September we traveled from Zagreb to Ljubljana on Flixbus. Our bus was stopped at the border and our passports were checked. Not a big deal just took a little more time.