Learning Multiple Languages At Once?

I've heard pretty strong cases for both sides of this argument, but what's your opinion? Is it better to only focus on learning one sole language at a time, or is trying to improve on a couple a time an effective method of learning too?

For me, I typically have one dominant language I'm spending the majority of my language learning time on, but other languages never completely go on the back burner. There will still always be times where I'm listening to Croatian music for instance, or watching a French YouTuber, so there's always at least a little bit of learning happening in a few different languages.