I *believe* it gets two wood materials from the blocks, and two from the boards used.
Note that you need a significant presence of a second wood type to get a result. 59 blocks of poplar and 1 of cherry will, for example, mean that poplar is used for both materials set by the woodblocks. A 35-25 ratio (Not entirely sure where the cutoff is) will mean that one of the materials is set by poplar, and the other by cherry.