Print artefacts when printing multiple colours

This is my first experience with using multiple nozzles, so I'm wondering whether anyone with experience of the Tricolor Mendel can help. I have an Ormerod which I've converted to dual nozzle. A problem I am seeing is that the non-printing nozzle is interfering with the printing nozzle. Where the non-printing nozzle crosses an area that the printing nozzle has already printed in the current layer, it either pushes the filament out resulting in a rough protrusion, or it pushes it in resulting in an indentation, depending on the direction of travel. In this photo, the white bands have indentations on the left, and the middle purple band has a protrusion on the right.



Can anyone suggest how to avoid this problem? I'm printing PLA at 180C, with the standby temperature of the non-printing nozzle set to 140C. The two nozzles are 14mm apart.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].