Lol jk but seriously trading codes is never a good idea. Its so easy to redeem them and then blocking the other guy or giving a dud code etc
I did think this initially but the accounts are very old and have high karma. I can't see someone risking their accounts for that. But who knows.
Not saying this is what is going on but i have my account which is this one and my brother has his account,i can log into both and they both have karma built up and were made a while back.Also people make alts and they too age and gain karma.
I honestly hope this is not what has happened here.