Animate spells don't need a target (when cast, or when returning into play during a WGD loop) - they simply trigger when they enter play, and if there's at least one creature in any graveyard, they will target that creature. If there aren't any creatures in any graveyards, then nothing will happen - the Animate spell will remain in play as a global enchantment, doing nothing.

So when two animates return along with the Witness, both animates trigger and are put on the stack (both targeting the same creature). When the first trigger resolves, that animate will end up enchanting the WGD from the graveyard. Then the next animate trigger will resolve but the creature it targeted is gone; therefore, that other animate will have no target and won't bring back anything.

The loop will begin again, but with TWO Animate Deads (One is already in play on the Eternal Witness you just animated, and when it is removed by the Worldgorger the Witness will just go back to the graveyard, ready to be animated again) you can Animate the Eternal Witness with the first and get a card back from your graveyard before you target the Worldgorger Dragon with the second Animate to clear the permanents from the board again. In this fashion, you can bring your entire graveyard to your hand. Since you can also play instants as the combo runs, you are free to recur Ancestral Recall with every loop of the combo and draw out your opponent for the kill.