The URL field only holds one link, if you try to put more than on in there it will treat the whole string as one link.
However, you can put as many links in the notes as you want, either in one note separated by spaces or as individual notes.

Ok, I guess that is an ok workaround, although having a shortcut to add multiple URLs and especially to open multiple URLs would be ideal.

The way I tackle this issue is to use OneTab (https://www.one-tab.com/), which is very convenient in my experience.

Ranbarton, that's a great idea. I've been using onetab but hadn't thought of using it for that.