Weren't the 1st and 2nd Hokage sealed forever?
All four Hokage had been sealed with the Shiki Fuujin:  


As you said, the First and Second Hokage had been sealed by the Third, in his fight against Orochimaru.  
On that same fight, the Third sealed himself, along with the First and Second.  
The Fourth sealed himself when he sealed the Kyuubi inside Naruto.




Those whose souls were sealed by the Shiki Fuujin cannot be summoned or reincarnated.
However, as we discovered in chapter 618, the seal can be broken by summoning the Shinigami while using a Shinigami mask that was stored in the Uzumaki Clan's Mask Storage Temple.