both groups want to expand the power of Islamic Theocratic Rule
Sure, but they have pretty significant divergences in their beliefs about Islamic jurisprudence. IS-K (IS Khorasan) is heavily populated by Salafists, who the Taliban (who are mainly Deobandis) suppressed prior to the US invasion. Now that the US is gone, they’re back to beefing.
But also, ISIS-K’s explicit goal is the establishment of a caliphate across the Khorasan region, which includes Afghanistan, and obviously the Taliban isn’t down with handing over power to them.