How do I modify the "Number of Pages" setting for an existing Master Page via javascript?

I'm learning javascripting for InDesign and I have been tinkering with the stock scripts in the tutorials. I'm developing the ability set properties and such, playing with how Javascript works, and I wanted to change the number of pages in a master page as part of an exercise. Does anyone know how to do this? I have been able to add new masters, controlling the number of pages in them with this:

myDocument.masterSpreads.add( {pagesPerSpread:2,} )

but what I really want is to modify the master that InDesign automatically generates with a new document, to have 2 pages. Granted, I can just start off with a document with facing pages, but I don't really see the fun in that...