Is this what you want?
You select Mac in the main category field. When you access the value list in the category 2 field, you see Macbook, Macbook Pro, and Macbook Air listed. If you select iPod, you get Nano, touch, shuffle in the category 2 field's list. If you select BOTH, you get all 6 values....
This is essentially a conditional value list with an added detail that you can select more than one category and want to see a combined list of items when that happens.
Here are three links on conditional value lists you can read and examine to learn more:
Forum Tutorial: Custom Value List?
Knowledgebase article: http://help.filemaker.com/app/answers/detail/a_id/5833/kw/conditional%20value%20list
The demo file should be of added interest for your problem as it demonstrates using a check box group formatted field to enable selecting multiple values in the category field to produce the combined list that it appears you want here.
Feel free to use Post A Answer at the end of this thread to post additional questions after you access the above threads and demo file.