I learned Korean and Thai at the same time and I didn't notice it was faster than learning, say, English by itself.
I think it depends on your personal psychology. If you're the type of person who doesn't get disheartened, I would say learn multiple languages at once(But be careful) I'd recommend learning from different language family trees. Say French and Chinese, as opposed to French and Spanish.
If you need to see daily gains to stay motivated then focus on just one language.
In terms of a learning schedule, come at it from different angles. For my Korean, I used Ling as a base because the app holds you accountable for daily learning. After that, I watched a lot of Netflix, read articles, and had a weekly lesson with a Korean-speaking teacher from Preply.