From the Pens twitter:
One call up was expected, since defenseman Matt Niskanen remains ill and will not play tonight versus the Ottawa Senators. However, bringing up the second player might mean that Kris Letang is out tonight as well. Letang did not practice yesterday and saw a doctor, as result of a hit from Edmonton's Gilbert Brule. Brule was penalized two minutes late in the game when he hit Letang high and late, Letang would not return to the game.
Potter, a former New York Rangers pick, has appeared in 8 NHL games, all coming with the Rangers. He's having a solid season in Wilkes-Barre and also played well during training camp this fall in Pittsburgh.
Wagner also has some NHL experience, with 46 games played with the St Louis Blues.
The Pens are in Ottawa tonight and then will be off until Sunday, perhaps a good time to rest Letang. However, it remains to be seen if the second call up is just for insurance if Letang can't go.
Letang is one of two players who have appeared in all 70 of the Penguins games this year. If he is out tonight, Max Talbot will become the only player to suit up for each game.