A single workflow should run on a single thread. That comes not only from the coding of CVW but also from the workflow design. Imagine the situation that an action is transitioning from one step and another is transitioning from another step or from the same step. It could mix up the workflow totally and the final result would depend on the execution order of the different threads (racing).
Action functions should be multi thread. When an action function (conditions, prefunctions, validators and postfunctions) is neede the simple implementation of the API instantiates an object and registers it in the workflow engine. The engine next time uses the same instance for the same class.