Hi,
I need to have my line snapping as I progress to draw it, to only those curves on a particular layer, whilst still being able to see other objects (otherwise I could turn off all layers except the two I am working with).
You could Ctrl-select the layers you’re interested in in the Layers panel, right click > select objects, then use Pascal’s Isolate script (specifically IsolateLock) so that only those object layers are editable. If you go to Rhino Options > Modelling Aids, you can uncheck Snap to Locked Objects - as clement says.