Void Aegis can be unlocked via either Future Mind or Eternal Form. However, with a point in each of the prereqs, neither one of the nodes can be respecced: when you try, you get an error “Cannot respec point. You have a point in Void Aegis, which requires you to have at least a point in it.”
Of course you can respec anyway by taking all points out of Void Aegis first, but that shouldn’t be necessary: respec logic should verify whether the requested build is valid via alternate prereqs and allow the respec if so.