Currently you can only have one set of tabs in each Dialog, possibly with the option 'extra #'
Code:
tab "Tab 1", 1, 5 5 100 100
tab "Tab 2", 2
tab "Extra 1 Tab 1", 5, 10 10 80 80, tab 2 extra 1
tab "Extra 1 Tab 2", 6, :tab 2: extra 1
tab "Extra 2 Tab 1", 3, 5 5 100 100, extra 2
tab "Extra 2 Tab 2", 4

Which looks easier to understand if i put it like this
Code:
tab "Tab 1", 1, 5 5 100 100
tab "Tab 2", 2 {
  tab "Extra 1 Tab 1", 5, 10 10 80 80, tab 2 extra 1
  tab "Extra 1 Tab 2", 6, :tab 2: extra 1
}
tab "Extra 2 Tab 1", 3, 5 5 100 100, extra 2
tab "Extra 2 Tab 2", 4

I'm unsure if the :tab 2: is needed bacause the 'parent' (tab "Name", ID, x y w h) tab already defines if it is in another tab
Which would look like this -

So the 'extra #' is the ID of that set of tabs