Dependent drop down list



I have a mapping module in one of my models, where I want to map a sub-scenario to a scenario. I have one dimension in my module, a list called "Scenarios". I have one line item, "Sub-scenarios" where the format is a list-subset to a list called "Sub-Scenarios". The sub-scenario list if below the scenario list in hierarchy. 

Now to the problem: I want the subset of sub-scenarios to be dependent on the scenario, so that you can only map a sub-scenario to its parent. At the moment, you can choose any sub-scenario to any scenario, which obviously is an issue. 

Does anyone know how to tackle this? Any help is appreciated! 

Thanks a lot!