He means if you use XYZ.com hosting for the .ro website and ABC.com hosting for the .cz website it is not possible for the .htaccess file from the .ro website to have any effect whatsoever on the .cz website... Even if they are in two different accounts at the same host it is not possible for the .htaccess file from one to have any effect on the other.
The only way it could possibly have an effect is if they are hosted at the same host and share the same file-system. The only other way the .htaccess file could possibly effect both is if you uploaded it to both websites.
What you are describing, by saying they are hosted in two different places and one .htaccess file caused both sites to stop working is like saying you uploaded an .htaccess file to your server and somehow *my* website doesn't work now, because you put a redirect from your site to mine in your .htaccess file... It's not possible.
There are three possibilities I see:
1.) You uploaded the .htaccess file to both websites.
2.) You redirected to the wrong page on the .cz website. (IOW You redirected to a non-existent page and for some reason are not receiving a 404 error, which means you have an issue with the .cz hosting.)
3.) There is something wrong with the page you are redirecting to on the .cz website and it's not loading.
The only way the .htaccess file should be the problem is case #1. You uploaded it to both websites. Otherwise it is most likely the page you are redirecting to is malfunctioning on the .cz website and it needs to be fixed.
Empty your browser cache.
Type the URL you are redirecting to into your browser.
See if the page loads...
If the page loads, copy and paste the URL to the .htaccess file to make sure you are redirecting to the correct page on the .cz site, then re-upload the .htaccess file on the .ro website
If the page does not load, fix the page.
If somehow you uploaded the .htaccess file to both servers, remove it from the .cz server.
Those are the only 3 possibilities I can think of.