Adding a Bowden extruder to a direct drive setup I currently have a single extruder (direct drive) Tronxy X8.

I am thinking about adding a second extruder, that I would use only occasionally. I really like the characteristics of the direct drive extruder, so I would not want to switch over to dual Bowden. Also adding the weight of a full second direct drive extruder for only occasional use does not seem useful.

Is it possible to use a direct drive extruder as main extruder and a (possibly detachable) bowden extruder as secondary extruder?

Is there anything in particular that I need to watch out for when mixing extruder types?
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.