The NFL released its schedule today and the Redskins have 3 games that conflict with the Nationals, only 1 of those dates will have both teams playing in Washington at the same time, the Thursday September 4 opening in NY (Nats play in Atlanta that night), September 14 home game against New Orleans (Nats are in Florida that day), September 21 home game against Arizona (the Nats are home against San Diego).
Other than the opener in NY the Skins have two national games, Monday night November 3 at home against Pittsburgh and Sunday night November 16 at home against Dallas.