By hand it is easy. Just use the output of the first blender as input of the second blender. The output of the first blender node is like a new texture where you put the second effect on.

Automating it with different combinations is a challenge

 If you want to see very complex ones of this method. Go to ShareCG and look for red_viper. He has easyshader3. This one adds scatter, wet skin, tanliines, blushes, soap and other things together.