Building up a terrace

I have a sandstone terrace (made from sandstone slabs placed vertically in the ground and then cemented together).

I wanted to raise the height of this terracing (so that the land above it will be flat). I was wondering where there was a technique to increase the stability of the structure (i.e. so that when I have two slabs on top of each other that they don't break at the join)?

It sounds Like you are building a retaining wall which is a lot more complicated than just putting slabs on edge.

Have a look at this link: http://www.anewhouse.com.au/?p=1585

The wall isn't really going to be that high - it is perhaps 15 cm at the moment and I wanted to add another 20 cm on top of this. Would bondcrete help to bind the two slabs together or would this not be sufficient?

No bonddcrete won't make any difference, the pressure only a small wall can develop is considerable.

The only reason councils aren't interested in smaller walls is because nobody is hurt when they fall over. I see plenty of brick walls 3-400mm high that are falling over every day at work.