The guy at the center of this said that the idea in their marketing strategy generally was (1) to consolidate their base, rather than to attract/detract from Real or Atleti supporters ... then (2) to carve out support amongst a possibly empathetic base. Second-choice support.
It is more complicated than merely Real or Atleti. When Getafe entered La Liga, there were FIVE Madrid-area teams competing in the league.
They did a lot of vetting before going pubic public with this.