Uh - no you can't assign two materials to the same polygon.

Once you use the group tool to change the assigned material, those polygons are no longer part of the previous material zone. Effectively you did nothing but make another zone that needs to be set.

However, many "dual" materials can actually be built with nodes. Have a look at this rather extreme example:

It looks like there are two surfaces, one that is guts, and the other that is gel. But it is all one surface, and just one shader.

To create the illusion of a multi-layer shader, you need to use Blender nodes. Think about your material in separate terms. Design each material by itself.

Then you must choose a pattern to overlay one material on the other, unless they are both uniformly present. In the case of hairs, you would use a control map just like a transmap. 

Now examine your two shaders. For every main channel that is different, you put a Blender node into it, controlled by your map. On the two inputs to the Blender, you put the two different values or nodes that you used in each of your separate shaders.

If you do this for every input of the Surface, you effectively create a double material. The control map (like a transmap) will choose one material or the other for you at each point. Of course if the control map is not between 0% and 100% then you will blend the two shaders. It is the same effect as partial transparency. For example, I did precisely that to create a dual shader that simultaneously mimics human leg skin, as well as nylon stockings over the skin.

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