... can mean both could be star born.. Or since both have some of the right aliments of the stars, with two tickets he increased his chances of winning the star born lottery..
As I so often do, I'll chant the magic words, "I don't think we have enough information to be sure."
My best surmise is that
Justin had no way to be certain, by himself... But maybe Mab does? Maybe Justin bought "two wizard-children, Starborn" from the fae.
Or maybe the Outsiders
themselves know? Harry believes Justin summoned HWWBehind, and if that's correct, Justin could have been following Outsider info-sources.
Or, as you say, maybe he was just playing
Starborn Lotto. Birth certificates would list date&time of birth.