How does one restore the twins in FF2 or FF3?

You can’t restore them, but Minwu (the Elder of Mysidia) heals them near the end of the game. While they don’t rejoin your party in the original version, they do become playable again at this point in the Complete edition.

P.S. It’s Final Fantasy IV that became Final Fantasy II in North America, not Final Fantasy III.

I was glad we got to see them again at the end because I thought that was one of the saddest parts of the game when they died. Hopefully my answer doesn’t get deleted but the person above me took care of the info quite nicely.