I don't think we will really know much until it is a common thing for reprap to use two tool heads.
here are a few outcomes
use 2 different materials one as a support material such as stratasys.
same materials at different temps
use transparent materials, materials of several colors
a mill that eats excess plastics away,
I think the big bang for the buck will be producing transistors/memory/photovoltaic cells and other electrical devices. several small print heads, and one big plastic print head assembly. so i would say piezo tool head in future. IMHO.
here is a possible example of a dual head printer making a compact antenna! ( Google'd it)
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engineering.illinois.edu]