I could be wrong about 5e's mechanics, but I'm pretty sure there are two things messing with your math here:
1) you can only re-roll 1 die. so if you roll 1 and 1 on 2d6 you only get to re-roll one of those dice and...
While that's a viable interpretation, I don't think it's that clear.
"When you roll a 1 or 2 on a damage die for an attack you make with a melee weapon that you are wielding with two hands, you can reroll the die etc. etc."
Not "once per attack." Not "If one of the damage dice is 1 or 2 when you roll damage." If X, then Y. If you roll 1 or 2 on a damage die, then you can reroll.
If you roll a 1 and a 2, each die meets that qualification.
Again, I don't know what the rules as
intended are, but I don't think the actual phrasing specifies only one.