So simplify3d ist mostly the best slicer out there, but when it comes to Dual Printing it not very good.
The main problem is the priming of the nozzles. i usually prime the nozzles with a skirt. but that does not work because my printer is always trying to print the PrimePillar/OozeShield bevor the skirt. so if the base of these 2 is not perfect the print tends to fail.
you need go give us more control of the hotends (set priorities of the Skirt/brim, Primer Pillar etc.). or a extra Priming function to prime the nozzles bevor anything else.
also in my opinion the prime tower is also very useless anyway. what the point of priming the nozzle that already printing anyway and cook the filament of the second nozzle in some cases for hours.
it should print with Nozzle one and prime Nozzle two so its not cooking the Filament.
it would make much more sense to give the option to prime the nozzle that is not printing atm so the filament does not cook. some people might disagree and in some cases it makes more sense to prime the other nozzle first. but you should give us more control about that.