Currently, the vertical double slabs quark adds can be stacked into double slabs like normal vanilla slabs can. However, there only exists one double slab block state per vertical slab block, which means that for example the slab/stairs fix resource pack I showed off recently on discord isn't capable of actually displaying the vertical double slabs in a consistent way.

An easy fix for this would be to split the "double" value into something along the lines of "double_x" and "double_z", so direction could be preserved and unique textures applied.