The beauty of learning something in your spare time is that no one will judge you if you decide you're not feeling it and bail. I think it's worth it to just pick one (Go if you want to learn a fast, compiled language that's pretty different than the dynamic-ness of Python, and Ruby if you want to explore a really fun-to-write general purpose language that is used for web, systems automation, and scripting mostly). If you decide you're not feeling it after a few/dozen/hundred sessions, just switch!
It's not like you're going to lose any of that practice/experience with whichever one you chose first if you end up switching. Any time you spend learning instead of trying to decide what to learn is valuable time gained, regardless of what language or topic it is.
That being said, try Go if you want to expand your toolset and have a fast language under your belt for if Python is too slow for something. Try Ruby if you want to do similar things to what you already know how to do in Python, but with a very different dialect/mindset that might change the way you think about writing your Python code too! P.S. I started with Python and still love it, but Ruby ended up taking its place as my favorite language to write. :)
I used to stress out that I was not focusing enough on a specialty, and was constantly jumping from thing to thing. Focusing is good to a point, but at a certain point, your free time is your free time and if you want to take a day to mess around with Haskell or something, don't feel bad about it! Some days you are allowed to Treat Yo Self!
It's really inspiring, I think, regarding all comments, that I'll give a shot to both and continue with the one I have the most fun :)
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