As it doesn't seem that they will always proc when off cooldown (You can LA to proc one, then LA attack again, and your other enchantment may not proc until the 3rd or 4th swing rather than always on the 2nd (even when it is on cooldown))
The Main-Hand enchantment isn't always the first one to proc (this was noticed after several instances of proccing the enchantments followed by 5s rest)
Nor is it tied to which hand you hit with.