Grouping items in a ComboBox

I have a ListView that contains two types of objects, single and multiple. The single is a ordinary TextBlock while the multiple is a ComboBox with items.

I'm trying to group the items in the ComboBox without success. Is it possible? Or should I go for a different approach?

What I'm trying to achieve:

[ComboBox v]
    [Header  ]
    [    Item]
    [    Item]
    [Header  ]
    [    Item]