You must choose two targets. You can't cast Victimize targeting only one creature card.

The creature you sacrifice isn't chosen until Victimize resolves. You can't return the creature you sacrifice because it will still be on the battlefield at the time targets are chosen.

As Victimize resolves, you must sacrifice a creature if able. You can't change your mind and choose not to sacrifice anything.

If one of the targeted creature cards is an illegal target (for instance, because it has left your graveyard before Victimize resolves), you'll still sacrifice a creature and put the other card onto the battlefield. If both are illegal targets, Victimize will be countered. You won't sacrifice a creature.

Victimize can be used to put both parts of the B.F.M. onto the battlefield at once.