Well, It's not very beautiful when you have two staves with slurs having the slurs cross together.
For example, I've got a violin II part ging from middle B to low D then high D and an alt part (in treble clef) staying between high D and G.
Both are slurred and the slurs cross together...
Eventually, NWC2 will allow some control over this, but for now:
https://forum.noteworthycomposer.com/?action=dlattach;topic=5873.0;attach=181 (https://forum.noteworthycomposer.com/?action=dlattach;topic=5873.0;attach=181)
shows an effective, but time consuming workaround. Set Highlight 2 to "white" as explained in: https://forum.noteworthycomposer.com/?topic=5676.msg36973#msg36973 (https://forum.noteworthycomposer.com/?topic=5676.msg36973#msg36973)
when viewing this file.