Did you notice that two different types of slabs are sticked together into one block?
This mod adds glue, which can be used for sticking slabs together. As of now, only the slabs from vanilla Minecraft can be glued together, others from other mod are not supported.
The recipe for glue:
(The recipes are unshaped)
The recipe for sticking slabs of the same type:
(The recipes are unshaped.)
The recipe for sticking slabs of different types:
(The recipes are shaped.)
You can also check recipes using Just Enough Items / Not Enough Items (just type "@mixedslabs" in the search box, without the quotes).
Note that I did this in 2 days, beware of bugs. Tell me if you find them ;) The mod is actually less buggy than I thought. But still, report to me if you found some bug(whether important or not).
You are allowed to redistribute this mod as long as you credit me and provide a link to here.
Folks at #minecraftforge, for helping me, and
of course the people working on Minecraft Forge.