I’ve asked myself this same question when I was learning both Esperanto and Spanish at the same time (and a little bit of Dutch). While I don’t have any scientific proof, I can talk about this topic from experience and what I’ve seen from polyglots in the community.
Learn two languages at once?
Think about this for a moment. Have you ever attempted to acquire knowledge about different subjects at the same time? For example, learning the piano and how to draw. It’s very easy to do. As long as you clearly separate the two time wise, you can learn both things. In school, you learn several things at once so why not languages? Is learning two languages simultaneously really that hard or confusing to do?
You might say, “well languages are too similar.” This is true, but ask people who do in fact speak two or several other languages and they’ll tell you how easy it is for them to separate the different lingos in their head.
When I think and speak in Esperanto, I feel totally different than when I am speaking Spanish. It’s like a whole different mindset. For me, it’s easy to distinguish the two and that makes it easy for me to learn the two languages at once.
There are even polyglots out there who learn many languages at the same time. One of those polyglots is Moses McCormick. You can see his YouTube channel here. He can speak somewhere over 40+ languages and studies two or more at the same time. His language skills are not bad either, when he talks to people he can actually hold decent length conversations. I’d say if Moses can learn multiple languages at once, then so can you.
Although it is easier to learn one thing at a time, if you really need to learn two languages at once or just want to, then I’d say go for it. It’s totally possible.