I started a game with a six person multi-player party. I have two fighters, and both rolled one for hp upon gaining second level. Now one reaches third, same thing happens. I go back to old saves, work back up till he is 3rd level again, but he always rolls a 1 for hp gained. Anyone knows what could be going on?

Low constitution maybe?

1st Fighters Con is 18, so he's always getting five hp (rolls a 1 +4 from con bonus), same with second fighter who has a con of 16.

It could be coincidence. When your character reaches level up, save the game before leveling. Reload until you get a higher HP score. Cheesy, but it should tell you if it's coincidence on the dice roll or not.