I have not had any success getting cura 3.2.1 with my z5fm2 which has the mixed extruder as well.
I have however had it work with repetier 2.05. I have only printed 1 dual color part, but what I did was slice it as a normal part. Then edit the G-code and add percents of each filament I wanted used at each level. I did not use a purge wall at all as I am terribly frugal... =) Hence the $200 dollar printer I had to wait a month for...
I just pasted this code at the layers I wanted to adjust
M163 S0 P80
M163 S1 P20
The 80 and 20 are the percents of each extruder. As you edit the G-code the layers are listed, so if you wanted to be scientific you could find the layers using the main view tools, then just paste your choices in after slicing. I was just making a cube different colors, but here is how it turned out.
I would love to try to get it working in Cura, so if you let me know what wizardry you used, I will give that a shot as well. Repetier does work, but I like being able to teach just one bit of software to the students I work with.