How to prevent websites from detecting you having multiple accounts in their system?

Me and my wife is having a problem with this gambling site where only 1 account is allowed per ISP. And if they suspected you having more than one account they will assume it as having multiple accounts, so how do we prevent it? I also have a series of questions.

1. Are Private Browsers with Tracking Prevention enough to prevent being detected of having multiple accounts in one ISP?

2. Can 1 computer with 2 ISPs (Network) any good in that situation?

3. Can simply clearing cache and cookies in your normal browser solve my problem?