I use Python and JavaScript at work and use Swift to develop my programming language Zolang.
Though If Python and JavaScript wouldn't be so trivial and similar with regards to code architecture + design it would probably mean that I would me merely a jack of these trades.
I think it's going fine, though I find it difficult to keep up to date with the latest greatest frameworks out there, especially in JavaScript and Python as they both have an insanely fast moving community.