Antonym of coterminous, or single word for "having the same beginning"
coöriginal or cooriginal (not comparable)
(philosophy) Existing together ab initio; equiprimordial.
(geometry, of a line) Originating from the same point in space.
Wiktionary
Consider "concomitant" and "coincidental."
concomitant: happening at the same time as something else.
coincidental and coincident: existing or occurring at the same time; occupying the same space or time.
To know the truth and to begin to know that you know the truth are coincidental.