That isn’t really the best workflow for this anymore. You can change to another number of lines (except 0) using Edit Instruments in D5. If you’re still on D4, then it’s better to use a doricolib file for this than to edit the instruments.xml directly, as all the factory files in that folder get overwritten with any update. @benwiggy posted a doricolib file with 0 and 1 line instruments here in this thread.
There’s not really any documentation for creating doricolib files, but they are very powerful and I use quite a lot of them to customize all sorts of aspects of the program. You can open Ben’s file though and get an idea of how they work though. Just a warning, if you get the syntax wrong, Dorico won’t start, so just be prepared to undo or delete doricolib files if you make a mistake somewhere in there.