I’ve personally only had experience learning two languages at once in the sense that I live in America, but my family often speaks Georgian at home and I studied Spanish throughout school, but I didn’t start Spanish until I was 11 or 12 so I had already solidified a lot of Georgian knowledge.
I think jumping headfirst into studying two languages rigorously/academically is harder though. You’d have to be really dedicated and in a rush for a specific goal. I think that talented, driven individuals certainly could learn multiple languages at once, but most of the time, I think just focusing on one, nailing it down, then moving on is better. I didn’t start Japanese until my Spanish was already pretty solid, but I still sometimes mix some Japanese words when I speak or think in Spanish (probably because my consumption of Spanish to Japanese material really started to go down after getting deeper into Japanese. Later on, I’ll have to refresh a bit I think).
Plus, even though there are some advantages with the kanji, it’s a double-edged sword. Now you’ve got completely different readings already competing for recall along with the (potentially) several existing Japanese readings (counting kun and on, since on is still more of a bastardized Chinese reading). So if you first get a solid grip on one, you’ll probably be less likely to juggle them with difficulty at the beginning. That being said, perhaps you’d pick up a strong habit for one or the other that might make getting accustomed to the second one more difficult.





