ok, so after further testing it appears that I still have the same problem. As this does block off unused space on each side, Cura still blocks off an additional 18mm for the distance between my two extruders.
Also for future reference the polygons on each side are not equal size. The left side would need to be just 4mm and the right side would be 26mm.
In Repetier you have the ability to set the print bed size as well and X min/max and Y min/max and somehow those setting transfer to the version of Cura used in Repetier. Anyone know how Repetier does this and if so can we make it work with Cura 2.7.0 or 3.1.0?
Any other ideas?