Bud Griffin published a new build:

Read more about this build...

 

Actually both extruders will have access to the entire build area. Only the extruder in use at the time will be in the build area. Each extruder will park itself on it's respective side of the printer. ReprapFirmware on the Duetwifi takes care of all the control of the extruders position based on the sliced model. Slic3r is able to slice a model to take advantage of multiple extruders.

From the Duet website on configuring multiple X axis:

Of course the true test will be in getting it all configured. But then where's the fun in it being simple.

 

The bed mesh is based on Tool 0. Tool 1 height needs to be matched with Tool 0 and then fine tuned with a Z offset.

Here's the link to Duetwifi concerning multiple X carriages.