No no the first one works but it's not what I'm looking for, I just want a mod that lets me put 2 different types of slabs on each other I don't even see that option on the first mod as for your mod pack I haven't tried yet.
While I'm not entirely sure about the slap on top of slab part, qCraft will do the other bit of what you want...kinda. It's a bit difficult to explain how it works (will put in a vid link at end of post) but basically you can make blocks that look different from different directions. If it is seen from one side, it would appear to be stone, from another it could be oak planks, and from yet another it would be air (complete with ability to walk through it). Perhaps not quite exactly what you're looking for, but fairly close.
Tis an old video, but is still accurate so far as I know.
Edit: as this post seems to be about 1.7, my information may be less than accurate.