Is it ok having both Anacondas 2.7 and 3.5 installed in the same time?

I use both depending on who in my department I am helping (Some people prefer 2.7, others 3.5). Anyway, I use Anaconda and my default installation is 3.5. I use environments for other versions of python, packages, etc.. So for example, when I wanted to start using python 2.7 I ran:

 conda create -n Python27 python=2.7

This creates a new environment named Python27 and installs Python version 2.7. You can add arguments to that line for installing other packages by default or just start from scratch. The environment will automatically activate, to deactivate simply type deactivate (windows) or source deactivate (linux, osx) in the command line. To activate in the future type activate Python27 (windows) or source activate Python27 (linux, osx). I would recommend reading the documentation for Managing Environments in Anaconda, if you choose to take that route.

Update

As of conda version 4.6 you can now use conda activate and conda deactivate. The use of source is now deprecated and will eventually be removed.

My understanding is you don't need to install Anaconda again to start using a different version of python. Instead, conda has the ability to separately manage python 2 and 3 environments.

Yes you can.

You don't have to download both Anaconda.

Only you need to download one of the version of Anaconda and need activate other version of Anaconda python.

If you have Python 3, you can set up a Python 2 kernel like this;

python2 -m pip install ipykernelpython2 -m ipykernel install --user

If you have Python 2,

python3 -m pip install ipykernelpython3 -m ipykernel install --user

Then you will be able to see both version of Python!

If you are using Anaconda Spyder then you should swap version here:

If you are using Jupiter then check here:

Note: If your Jupiter or Anaconda already open after installation you need to restart again. Then you will be able to see.