Can you play Existential Dread on 2 units if you repeat it?

Yes. Existential Dread's [Repeat] [2] lets you execute its effect a second time, and each execution can target a different attacking enemy unit — so you can stun two units (or return one already-stunned unit to hand and stun another).

Both choices are locked in when the spell is finalized, so both targets must be attacking enemy units at that moment. Note that Repeat's cost can only be paid once per instance (Rule 820.1.c.3 / 746.1.c.3), so you get exactly two executions — not more. FAQ #11275 confirms the targeting for each Repeat execution is chosen independently.