I normally use python 2.7.3 traditionally installed in /usr/local/bin, but I needed to rebuild python 2.6.6 (which I did without using virtualenv) in another directory ~/usr/local/ and rebuild numpy, scipy, all libraries I needed different versions from what I had for python 2.7.3 there...
But all the other packages that I want exactly as they were (meaning same version) in my default installation, I don't know how to just use them in the python 2.6.6 without having to download tarballs, build and installing them using --prefix=/home/myself/usr/local/bin.
Is there a fast or simpler way of "re-using" those packages in my "local" python 2.6.6?
Reinstall them. It may seem like a no-brainer to reuse modules (in a lot of cases, you can), but in the case of modules that have compiled code - for long term systems administration this can be an utter nightmare.
Consider supporting multiple versions of Python for multiple versions / architectures of Linux. Some modules will reference libraries in /usr/local/lib, but those libraries can be the wrong arch or wrong version.
You're better off making a requirements.txt file and using pip to install them from source.