Well, ICO and Shadow take place a long time apart from each other, since Wander is the progenitor of the line of horned boys and by the time ICO takes place, there have been many, possibly hundreds of previous sacrifices.
If I understand it correctly, Ico himself was the 40th sacrifice, if you count by the pots in the first chamber. At 1 sacrifice per generation, that puts them 600-1000 years apart.
