One issue that became apparent during the first dual print was oozing. No matter how much I retracted the filament while the hotend was parked, there was always a tiny string oozing out. The dual print only finished because Cura distributes the starting point on the prime tower in each layer. Adding a really large prime tower with thin walls finally did the trick.

The current design clearly lacks a feature that stops oozing by blocking the nozzle in the parking position. The hotend fixture only provides close control of the pin positions but leaves a lot of slack for the nozzle, which makes it hard (impossible?) to block it. Experiments revealed that entering and leaving the parking position at the same location gives a high chance of picking up oozed filament that was wiped off the nozzle before. 

Other issues with the current design are

• the nozzle is slightly deflected by forces applied to the bowden tube
• no space for part cooling fan(s) left

Time for a redesign to address all of these issues. 

• unidirectional (left to right) load/unload, no risk of picking up oozed filament while unparking
• the parking fixture (yellow) now provides support at the top and bottom
• four instead of two pins/bushings to register the hotend to the carrige
• stress relive for the bowden tube
• space for two 30 mm part cooling fans

A small plug made out of 350°C silicone should block the nozzle. Not sure if I'm able to cast (more like squeeze) this type of silicone in a mold, if so, adding feature to wipe/clean the nozzle should be easy.