This node will allow the user to pick model elements in desired order while isolating category.
https://screencast.autodesk.com/Embed/Timeline/2953357c-9059-4629-b7cd-4d680d50a2ba
Source: http://sixtysecondrevit.blogspot.com/2016/04/isolated-pick-selection-in-revit-with.html
Dynamo Nodes 