Currently a package can have only a single version of a dependency defined.
I have a need to install multiple versions of a dependency at once.
This is a subset of aliasing features that were rejected in #2943.

In my use case I have a library (frozen-express) that can during runtime take an object produced by another library (Express app) and do useful things with it.
I support multiple versions of Express.

During development I want to be able to test my application with different versions of my dependency. I could fake both versions of Express, but I consider that using a real Express app is a better deal. The tests become more of integration tests as opposed to unit tests, but that only makes them more useful.

I could use npm install express@3 && npm test && npm install express@4 && npm test, but that is very significantly slower than running my entire test suite.
In addition, this wrecks any chance of receiving proper code coverage reports.
Obviously, I have code that conditionally applies to one or another version, and no code coverage tool understands that, if I need to run npm test twice (or more times).

The best workaround is, unfortunately, abusing the npm database. A single-line module express3 that depends on express@3 and exports it solves all my problems.

Perhaps this feature request is a tiny bit more viable than #2943?