If it's more arcane than that, it sounds like kvm or lxc is the answer.
If you really really need to run two pieces of software with differing ruby interpreters on the same machine, then your real problem is technical debt. One fix would be to update the ancient software so that it can use modern Ruby. Another fix would be to re-architect the solution so that the greater system could operate across multiple OS instances. The least attractive option would be to use packaged Ruby for the modern stuff, and a hand-compiled Ruby for the legacy stuff.
Why?
Why can't you have a stable app using Ruby 1.9 that hasn't need any updates (other than security) in three years running on the same machine that also runs another newer application which uses Ruby 2.0 and Rails 4?
To me, that's not "technical debt".
Maybe rvm/rbenv is not the best solution, but some solution is needed. You can either invent one yourself, or adapt something that's already been built. We're using rvm, and while I'm not a huge fan of it and would rather switch to rbenv, I've managed to tame it so that it's not such a huge beast, using various wrappers so that non-interactive Ruby programs work start up in the correct environment.