Adding a Bowden extruder to a direct drive setup
I'm an amateur and I used dual extruder printers but never built any.

It's probably a good idea, take care about the purge, retractions distances that may vary. On the filament feed side there shouldn't be interference from the different flexibility. Still it maybe good to place one reel on one side and the other on the opposite. 

Now the most complex thing is to make both nozzle not interfere. A technique is to rotate by few (like 5°) the dual setup to give few millimeter more to one of the two nozzles. If both heads are at the same height, you'll need to keep them hot, and avoid the passive one to leak. It would be good also (but not mandatory) to have a purge routine.