How do I make 2 combo-boxes dependant on one another?

Hi Ashlee , this is for sure do-able.  I have an example of this I just did for a client, however instead of using Combo Box selectors, I'm using the button component.  Which has an advantage because the button component property for Button Type is 'Select Member' which I believe is more user friendly than a combo box if there are many, many selection options.

Button 1 - needs a dashboard extender rule function Server Task to apply the parameter selection to the next component.  It would look like this:

The key line of code there is the ModifiedCustomSubstVars action on the Selection Changed Task Result.

Button 2 - That parameter selection in the ModifiedCustomSubstVars I'm using as the Bound Parameter is button 2.

Lastly, the button actions need a User Interface Action of Refresh the embedded dashboard that contains the component selectors (Button or Combo Box).  Otherwise you won't see the update immediately in your UI.

Hope this helps,

Hi Ashlee 
Maybe post your configurations of the combo boxes? It may just be a setting...

This is a requirement that comes up on many projects.  It has the common name over the years as "Combo Edits".  Essentially we want to prevent a user from adding data to dimension intersections that are unrelated other then they exist.  
e.g. If a communication company has sales channels "Subscriptions" and "Equipment" , and Product hierarchies "Consumer Electronics" and "Streaming Services", you would not want a planner to have access to enter a subscription revenue data for a product like a Router that is simply sold once.

One way to accomplish this is with a lookup table that manages relevance. 

With a dashboard Extender rule, you can get the selected value of the first combo box and use that to query your table where you store the names of the hierarchy or members that are relevant  to the combobox choice.  The results of the query are assigned to the parameter of the second combobox.  This all gets passed back to the dashboard using the XFSelectionChangedTaskResult object.  Then the second combobox will dispaly only the values you looked up in the dashboard extender rule.

The first combo box will need a Server Task assigned that is the Dashboard Extender rule (DER) that does the lookup.
It will also need a bound parameter.
The DER will set the bound parameter on the second combo box.