Require One attribute of two

I've spent two days searching for this answer, which means I can't find it or more likely it's not possible to do.

I have a product that we have 'stock' options and 'non-stock' options. I need the customer to be required to choose a selction from one OR the other drop downs, but never both. If choosing from the non-stock options I need to put a minimum quantity required. Cost per item is the same.

Is this possible?

Example:

Handmade Bar soap:

Scent - In Stock (drop down list of scents)
Scent - Non-Stock (drop down list of scents, require min purchase of 4)

The customer MUST choose from one OR the other, but not both.

Is this possible? If so, please help a husband out with his 'customer' :blush:

Thanks
GERob