When you need two versions of the same process to be available in the engine at the same time, you should make sure both versions of the process are still available on guvnor. They should have a different id (but share the same name, just a changed version number, e.g. id's com.sample.evaluation-1.0 and com.sample.evaluation-2.0). That way, the process image associated with each of these processes will also be using a different name.

It should be easy to create a new version of a process based on an existing one.