During the Campaign when a hero trains a skill can another hero train the same skill because there is only one skill card.Or is it like the basic game one hero per skill card.

Laughinrob said:

RtL pg 23, Skill upgrades

Once the appropriate cost is paid, the hero player takes the appropriate skill card, as normal.

Just as in the base game, the hero now has the card, so no one else can have the card. There is no indication that this restriction has been lifted.