[question] Use multiple virtual environments

Hi, we'd like to use both virtualrunenv and virtualenv together when installing a package. We define some environmental variables in a recipe, which needs to be visible by a package consumers - this is done by virtualenv generator. Also we need to use virtualrunenv in order to configure envs like LD_LIBRARY_PATH etc.

I have not found info in the docs that following actually works: `conan install -g` only supports one virtual environment at a time, but you can pass multiple `-g` in the same command:

```
bash-4.2$ conan install package/1.0 -g virtualenv -g virtualrunenv
bash-4.2$ source activate.sh
(conanenv) bash-4.2$ source activate_run.sh
(conanrunenv) (conanenv) bash-4.2$ env
# LD_LIBRARY_PATH, PATH and other envs properly configured
```

My question is where it is an advised way to do so. If yes, do you think it's worth adding such example to official docs?

- [X] I've read the [CONTRIBUTING guide](https://github.com/conan-io/conan/blob/develop/.github/CONTRIBUTING.md).