No, I wasn't being sarcastic. If you want to use libraries written in Python in Ruby (just as an example), you need to be able to convert all the Pythonisms to Rubyisms and/or Rubyisms to Pythonisms. That is the case whether you are doing source-to-source rewriting or using a shared VM.

Note that the JVM and .NET runtimes do pretty much exactly what was asked--they run programs written in multiple languages on the same VM. Because the "native" VMs for Python and Ruby are so bad, the JVM and .NET VMs can even out-perform the native ones for most tasks.