Is it ok to install both Python 2.7 and 3.5?

I have installed two versions, 2.7, 3.4 and I do not have any problem by now. 3.4 I am using for my work project in eclipse environment, 2.7 for udacity course, like You ;).

As long as you keep your installation folders organized, you should have no issues having both on your computer, besides one thing. The path environment variable for python will determine which version is used by default, so I would say stick to one version, or make sure to make your programs as backwards compatible as possible. I have run into this issue on Windows, since I installed Python 3.4 before 2.7, and therefore to run older code, I have to manually select the python executable. In terms of libraries, I believe that for each python version, the libraries are completely separate, so you should be good there.