I have two domains for two different languages containing the same content. Example.si in slovenian and example.eu in english. I am thinking to use only one domain (.eu) and have both languages on there. What is the recommended choice?
It's really a matter of personal preference; there are pros and cons to each approach.
The biggest advantage to separate domains is visitors are less likely to accidentally see one of the pages in the wrong language, as the whole domain is in one language, but if a user ends up on the wrong domain they may not realize they could visit your other domain and see the same content in their language.
The biggest advantage to a single domain is you get all the SEO value to that one domain, but that can also be a challenge in that it may be harder to rank for multiple languages.
In either case, it's most important to make sure that you provide search engines with hreflang or some other canonical so that they realize that both pages have value and are just in different languages. This will help avoid duplicate content problems, where search engines might think you've just copied a page verbatim (especially in the case of separate domains).