I wouldn't recommend it for (even remotely) similar languages. But for totally dissimilar languages this should be fine. For example I'm currently learning both golang and erlang, and it's as efficient as learning them one at a time. But I wouldn't learn e.g. C and golang at the same time, or even python and golang at the same time.
Also, I wouldn't recommend learning more than one language at a time until you know well at least two - three dissimilar languages (i.e. at least two different paradigms, e.g. structural and OOP)
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