i have try to load master page by script (i post a small script test) but indesign crash... if i want copy masterspread from doc1 to doc 2 i must use "duplicate"...

target indesign

// create two document
var doc1 = app.documents.add()
var doc2 = app.documents.add()

// add new master spread and set it
var master = doc1.masterSpreads.add()
master.namePrefix = "XX"

// save document on disk and get file
doc1.save()
var doc1File = doc1.filePath

// close
doc1.close()

// load master spreads of doc1 on doc 2
doc2.loadMasters(doc1File)