You need a mask and blender(s).

A mask is like a transparency map, but you don't use it for transparency. You use it to identify areas of a prop or figure that belong to one of two different materials. You use black for one area, white for the other. 

Once you have a mask, you look at the two materials and compare them, in preparation for combining them. Where they are the same, the combination is trivial - use the value or node that is common to both. Where they are different, you place a Blender node, and set or connect the two values in Value_1 and Value_2. The mask goes into the Blender's Blending input, and set it to 1.

So - if you have two different color maps, blend them. The mask will decide where to use each.

If you have two different specular nodes, blend them. The mask will decide where to use each.

If you have two different bump maps, blend them. The mask will decide where to use each.

If you have the same node but a different number in a particular parameter, blend them - use two math nodes to hold each value and plug those into the blender.

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