Dropdown list

Ok lets say I have the following form:

Fields: Brand & Model
Data:
Brand Model
Nokia 5300
Nokia 5310
Nokia N95
Motorola V3

If I now create a new form and try to create a drop down button linked to the other form I see:

Nokia
Nokia
Nokia
Motorola

That's because Creators considers every brand is different. Now, I could create a dropdown list for the Brand & Model form, but then my users won't be able to add new brands.

So how can I do to create a dropdown list that we show the grouped results of the Brand & Model form? The expect result should be:

Nokia
Motorola

Thank you all for your help