the most consistent element that differentiated Dispensationalism from all other eschatological systems was that in Dispensationalism, God has a different redemptive plan for racial Jews - unique, separate and even superior to any other redemptive plan offered to any other racial group (Greeks, gentiles, etc).
Is this two peoples of God scheme a given (an axiom, unproven), or do they get it from somewhere?
Sure. Scofield Reference Bible, for starters.
For an IMO least-biased answer, I would refer you over to Frumanchu's thread Four Views On The Millennium. The article contains some excellent references and resources for further study re any of the four systems, dispensationalism included.