Summary

I've seen how much print quality increases with dual fan ducts.

I've also long been envious of the Ultimaker 2's dual fan ducts.

I'm no longer envious...

Instructions

Adding a second fan-duct and fan will require a few extra parts and some soldering.

Get a proper heat sink for the 12V voltage regulator on the UM motherboard. The extra current draw from a second fan will cause this part to heat up even more than usual. You risk burning it out without one.
Make (solder) a parallel Y connector to connect to the existing cooling fan wire at the hot end. Then the 2 fans can be connected in parallel to the branches. It helps to get some M/F connectors to make connection and disconnection easier.

Heat conduction to the fan duct is quite strong nearest the heater block. PLA will go quite soft due to the low glass-transistion temperature. ABS might be a better alternative for this application.
I'm stuck with PLA for the time being so I use a zip tie to reduce the sag. This approach has worked nicely for some time now.

Ultimaker Second Fanduct