Is that not already weird?Now add the extra weirdness that you can't know what either envelope contains until you open it at which point whatever you find in the first envelope you will find the exact opposite in the other envelope.
It's like if you had two dice, you can't tell what number is going to come up next just by looking at it. You roll the dice, which don't stop spinning until you focus your eyes on one die, at which point both stop spinning. You look at and note the result of the first die, then you look at the other die and it is always facing exactly the opposite of the first die. That's a bit weird.