Can a virtualenv inherit from another?

I want to create one virtualenv using another as the starting point, is this possible?

I have to use cases in mind:

1. Let's say I have a two virtualenv one for production and one for development. The development environment requires the same packages as the production environment, but it requires others I don't want in the production environment. I don't want to install the common packages twice.

2. I want to experiment with a development version of a package, say matplotlib for example. The development version of the package has the same requirements as the stable version. So I create a virtualenv called matplotib_stable and install the requirements and the stable version. Then I create a second virtualenv called matplotlib_dev and use matplotlib_stable as a starting point (for the matplotlib requirements) but then I install the development version.

How do I install from a local cache with pip? seems to address the issue of downloading packages, but I don't think it deals with modifying sys.path.