2P means 2nd Player, Like in a video game, if two people are the same character, the second player gets a recolor of that same character.
So for 2P Hetalia, its the same characters, recolored with different outfits and such.
Some fans speculate that in the 2P!Hetaia universe the chacters have a complete opposite personality. Ex: England would go from Cynical and grumpy, to happy go lucky and smiley, ALL the time. Italy would go from innocent crybaby to someone rather angry and violent.
It is also thought that the 2P!Universe is violent as well, thus explaining the blood in the picture.