Is it possible to use both python 27 & 34 on same computer ?

Answer 1

You can have as many versions as you need. Some libraries might only work with certain versions (either 2 or 3, although there are ways to fix that).
Also, you might think of having a 32-bit and a 64-bit version, if for example you want to convert your code to an executable (by using py2exe or cx_freeze).

I recommend installing PyCharm which can easily handle several different interpreters for your projects.

Answer 2

yes, you just need to specify which one you are using. I have both installed right now.