Personally I've tried it and it hasn't been productive.
What I have been productive in doing is learning one new language at a time and doing a compare/contrast. For example, my main language is Java, so when it came time to learn Python I went back to projects I've completed in Java and did them again in Python. After some time I wasn't sure how much I was feeling Python, so I hopped on over to C# and did the same thing. Now I'm back on Python, lol. BUT there wasn't a time where I was doing both Python and C# - it was one or the other.
That's my two cents.
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