I've noticed 3 things impacting CR-X dual printing with Cura 4.0.0 Beta (OSX):
1) Extruder Start/End Gcode isn't inserted on tool change. My workaround was two post-processing scripts, which is kludgy but works fine. Through trial-n-error I figured out that retraction on Extruder End Gcode needs to be more than -60 mm (Creality recommends -80) and re-priming after tool swap needs to similarly be >+60 (I used +80 again).
2) The default prime-tower-volume is WAY WAY too small to purge between tools. I had set it to >80 mm3 on each extruder and 90 might be safer. (FYI, I use a 30 mm circular tower). For reference, S3D profiles seem to use 91 mm3.
3) the tower often topples over so I use a brim whether I need it or not for the tower.