Dual printing can be very difficult if the 3D printer is not calibrated correctly.  If the printer heads are not at the correct height, they can drag through already printed plastic.  This leads to the wrong color plastic smearing in areas it does not belong as well as scar marks from the inactive heard.  If you are looking to improve dual print quality Airwolf has a guide that outlines proper leveling technique.  

Guide: https://airwolf3d.freshdesk.com/support/solutions/articles/13000007234-axiom-dual-nozzle-leveling