I looked at various configurations but settled on this centrally mounted 40mm centrifugal fan concept. The nozzle design clips firmly into the particular fan I chose and serves as a part-fan for both extruders, furthermore, it can be lifted out of the way as required.

I spent some time with smoke studies to get the right nozzle shape and it has already proven its worth printing thin-walled parts including its own nozzle (0.8mm wall). I have never used a part fan before but I can see now that it is an essential tool in the FDM tool kit! Easy to print with both exhaust nozzles on the print bed. I optomised the design for 0.4mm extruder nozzles with no infill, however, I recommend using a part fan to print this...
I have uploaded the original mounting bracket and arms which is a work in progress. I have also added an optional bracket to the end of the nozzel so that it can sit on the rail. I added the brim for adhesion in the solid model - obviously you need to cut this off once printed.
The assembly requires five M3 screws; A 20mm one that holds the fan, two 8mm long screws to secure the arms onto the top mounting bracket and two 6mm ones to secure the assembly onto the alloy mount.

Here is where I got the fan from; http://www.aliexpress.com/item/Plastic-10PCS-GDT-DC-24V-2P-Brushless-Cooling-Blower-Fan-40mm-40x20mm-4cm/32270564254.html