I have many models in my shop that have operating joints, etc .. all you have to do is design the model so that the shells overlap, but have a wee gap between them so as not to touch. You can check the various design rules to see what that gap needs to be.
All four axles on this model http://shpws.me/2zQ4 freely rotate.. the whole model is not an inch long.
One thing though.. some of these materials are not going to take rough use.. some of them like FUD are not able to withstand much pressure.
The other thing is that you can't do this with glass, ceramic, or metals.. they will fuse together during post-processing.
Shapeways does not at this time have facility to combine materials, nor to assemble various parts after printing. You'd have to do that kind of thing yourself.
[Updated on: Fri, 02 March 2012 19:41 UTC]
Patience, Persistance, Politeness - the 3Ps will help us get us to Perfect Printed Products