There is mentioned, multiple account and account sales are allowed.
Its mean, no problem if 1person having 2 account?
It is allowed but unless you publically declare it then it will be considered untrustworthy behaviour and you'll get a red tag. For example, theymos has a second account called theymos_away and everyone know that so he's not hiding anything. If someone has multiple accounts so they can cheat bounties it is another story.