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Is it possible to have an NSOutlineView with two triangles in one row, where each triangle expands a different data set?

Not without an undue amount of hackery, and it sounds like a UI nightmare to boot. I’d suggest setting the row to have 2 children (one for each data set) and then make the data sets children of those child rows. – Bo

Why would you want to do it at all? Chances are, it’ll confuse your users.