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.
I believe that when they detected that you created multiple accounts, they will at least give you a warning right?
I will not use the old one anyway and will focus on using the new one. Anyway, now that the "Reset Password" seems working already, I will try to reset the password of my old account and then delete it to prevent further issues in the future. I don't see the reason why people are creating multiple accounts. The bonuses, I guess?