Why don't you let OBIEE make the work by using the "limit values by" checkbox? As long as your model is correct OBIEE will automatically limit values of your second prompt based on the first one.
+ 1 to Gianni Ceresa
The usage of presentation variables in prompts works only if the selectors are in different prompts objects.
e.g. if you want to influence Product Group by the Brand selected you'll have to put Brand in a separate prompt.
But...You have the "limit values by" feature by default... why don't you use that?