If I select with Existencial Dread, repeating it's efect the only 2 enemy units at a battlefield, can he use repulse to defend one of this units from the stunt? Thank u!

No — Repulse can't be used here. Repulse only counters an enemy spell or ability that "chooses it and no other friendly unit," and your repeated Existential Dread is choosing two of the defender's units, so no single one of them qualifies.

Paying Repeat doesn't create a second spell: Existential Dread is played once, with both targets locked in at finalization (Rule 820.2.a, FAQ #11275), and Repulse counts every friendly unit the spell is choosing — the same logic as the repeated Bellows Breath rulings (#9655, #10127). Both stuns resolve normally.

The only way Repulse would become legal is if one of the two targets changed to a non-board zone (e.g. killed or returned to hand) before Repulse is played, severing that targeting relationship (Rule 359.3.e.9.a) — simply moving it to Base or another battlefield doesn't help.