The problem is when you use both account at the same time to abused casino features such as promotion and bypassing limit set on one account.
That's the main reason why casinos don't allow creation of multiple accounts. Although, if it works for someone then its okay but for ones safety it's always better to avoid creating multiple accounts at a casino. Personally, I avoid creating multiple accounts and I also save my account credentials on paper as well as a text document so I never forget about it.