You are saying, you believe it's only possible, to only combine two materials on a terrain.

There are so many possibilities in the advanced material editor ...

That's just the start of a terrain texture in Vue. Depending on the scenery you are going for, this is sometimes really enough, to show plenty of variety, if you are adding and use the extra options for terrain material influence. Like slope, height, direction settings - using a variable density and positioning through a filter, function or greyscale bitmap driven placement - hard edges, flowing edges between the materials - variable colors again controled by filter, function, greyscale bitmaps - or use of displacement - or change the surface in details and shape by using hyper-textures and, and, and ....

And - the two combined start materials are just the 'start' of your material line. Each of those two start materials can be edited separate from each other and make out of each of them again mixed materials adding in each deeper hierarchy step more materials, which can be controlled with the above mentioned methods to make up really extrem detailed and influenced terrain textures.

There is even more - each material can be made out of multi-layers

The possibilities to create very complex terrain materials are all there - they just have to be activated and put to good use. Sure this will take quite some time in handling and understanding how it ticks, but each program got it's learning curve and in my opinion with Vue i've had the easiest start compared with other 3D graphic programs i have tried before. And - i still don't use Vue by far not to it's fullest potential - even after 2 years of use.