You could use a Merge Shader to mix multiple heightfields, but it may not get you the results you're looking for. Frankly I'd probably do this instead in an external heightfield editor like World Machine or Wilbur, depending on what level of control was needed, etc. It's certainly possible in TG2, I'm just not sure it's the most effective way to do it. Basically TG2's strengths lie in rendering and procedural functions. When you start to get into heightfield manipulation, although TG2 has some capabilities in this area they really pale in comparison to dedicated heightfield manipulation tools, so unless your modifications depend on "runtime accuracy" (in other words doing this manipulation in an external editor and saving to a finite resolution heightfield would lose accuracy somehow), then I would recommend another tool for the actual adjustments, then bringing the terrain back into TG2 for rendering.
- Oshyan