I've got exactly the same question. Of course the two objects have to be exported separatly to have different extruders assigned to each.
The question is how to tell (force) Cura to start with the extruder that draws the text, and not the surface surrounding it.
When my UMS5 first draws the surface, and then tries to fill inn the text, the text shows up blurry. I find no place to switch this sequence.