A simple partial erase the arc should do it. Just tried it, arc, line across it, erase/partial erase 3 clicks on the intersection, and the arc to the left I could change to another color.
Roger Donaldson AIA CSI CDT NCARB abUsing Datacad since 1986, ver 23.00.01.02 (SE) http://www.RogerDonaldsonArchitect.com