Language forums are full of people asking whether they could learn languages A and B at the same time. The reason this question keeps coming up and is never settled conclusively is because everyone's situation is different. So instead of telling you whether you can learn several languages at once, I will guide you to figure this out for yourself.
There are two main considerations:
1. Interference
2. Efficiency
Let's take them one by one.
1. Interference
When you're learning two languages that interfere with each other, that is obviously bad. In the worst case, you won't learn either of them well and you'll be stuck in a perpetual hell of Itañol (to give the most common example - Italian and Spanish are incredibly susceptible to interference).
You should be worried about interference if:
a) You are learning two languages from scratch. In this case, even languages from different language families but with somewhat similar phonetic systems (e.g. Greek and Swahili) can interfere with each other until you have reached level A2 in one of them.
b) You are learning two languages from the same language family and with a similar phonetic system (e.g. Spanish and Italian), and you are not yet at level C1 in one of these languages. In the case of languages that are so similar, A2 or B1 is not enough in order to stop confusing them; you need to reach a higher level.
Quite simply, for each bit of similarity, you need to compensate by having a higher level before you start the second language from scratch. In the case of languages with very distinct phonetic systems, e.g. French or Chinese, you may be able to learn two languages from scratch at the same time without too much interference.
If you're particularly susceptible to interference, ensure that you are level A2 or higher in every other language before you start a new one - or, if you cannot be bothered to improve a certain language anymore, accept that your less-than-A2 language will atrophy in favour of the new language.
2. Efficiency
Surprisingly enough, learning more than one language at once can be more efficient than learning just one language. Often it is not, but sometimes it can be.
Basically, you want to spend the maximum amount of hours on your new language when starting out, so that you quickly leave behind the boring beginner stage and get to enjoy the benefits of the intermediate level. These benefits include: being able to have conversations about everyday life, understanding more and more conversations in movies or TV series, being able to read online articles with the help of a dictionary or reading tool, and having to put in less time into maintenance (the higher your level, the less hours per month are necessary to keep the language at a reasonable level).
So generally, studying only one language at a time should give you the maximum amount of hours: every hour that you have available to study languages, you'll be studying this language and not splitting your time between several languages.
BUT, there are two cases where this does not apply.
1. If studying one language requires a lot of mental energy (e.g. studying a textbook, Anki) and studying the other language does not (e.g. because you're at a stage where you can just watch a TV series or read a comic and learn vocabulary from context), then it is more efficient to learn these two languages at the same time. Whenever you have a lot of mental energy at your disposal, study language A. Whenever your brain is mush, watch a TV series or read a comic in language B.
2. If you are one of the blessed people with several free hours a day that you can dedicate to language-learning, you may get overloaded or bored doing just one language, so that you stop and don't actually study as many hours as you could. You can try to counter this by using a variety of materials and teachers to make study time more interesting, but if you often find yourself stopping and not studying anymore for the rest of the free time, having a second target language (that you're excited about) may be the solution. It gives language A time to settle and it spices up the day. Your total time spent on languages will be higher then.
My own system
I usually have TWO focus languages for each 3-month period. One focus language is a language where I'm a beginner and need to do a lot of hard study, and the other language is one of my intermediate or advanced languages where I do less brain-intensive activities. By declaring both of them to be my focus, I put in more total time, compared to what I'd be doing if my only option was to study my beginner language - in that case I'd not do anything when my brain is mush.
Why 3 months? It isn't terribly long to put off the other shiny languages that call to me and it gives me the time to get to a decent level in them. I find that it is usually easy to maintain motivation and discipline for that long but harder later.
A recent example: for the past few months, I did a 90-day challenge to learn basic Russian from zero and simultaneously enrolled in the Super Challenge with a goal of reading 50 books in Serbocroatian and watching 50 movies in Serbocroatian by the end of next year. (Going in, my Serbocroatian was level B2 already.) This means that when I had the mental energy, I'd work on Russian, and when I had less/no mental energy, I can still put in some hours towards Serbocroatian by consuming some material in that language and counting it towards the Super Challenge. As for the C-level languages, I don't "study" them. In my experience, I need about 20 hours / year of maintenance for each C-level language and these are easily achieved without paying particular attention to hours, just by enjoying conversations, books, movies, news, emailing with friends and colleagues... When a language is well-integrated in your life, 20 hours is nothing.
Your specific case
Think about how the above applies to you:
=> If you expect interference because the languages are too similar and neither is at a high level yet, stop thinking about studying them at the same time and rather focus on upgrading the one you're better at - unless you're ready to lose that language.
=> If you have several hours a day and you often get bored before the time is up, you're cleared to study more than one language at the same time; it's probably more efficient than studying just one.
=> If you have limited time and you are planning to study two beginner languages, you'll be progressing at half (or less than half) speed and much more likely to lose motivation before you've made visible progress. Don't do it.
=> If you have limited time and you would like to study one language through brain-intensive methods (textbooks, Anki, classes) and another language through low-brain activities (watching TV, reading stuff), go for it! Just be sure to actually do the hard study whenever you have the mental energy.
Good luck with your studies!
