I know of two people that opened multiple betting accounts but their interest was not exactly having multi-accounts to play but to try to arbitrage spreads when they could find a game or a bet that had a very tiny spread or even a "sure bet" by betting opposites in both houses. These multi-accounts tend to last only a few months because the betting houses tend to catch and ban the creators.
^ I have a little confusion in my mind on how they can catch the creator of multiple accounts, the same owner on different devices will most likely hard to catch it. It should always be good to read first the TOS before using the casino, some casinos don't mind having multiple accounts as long as you did not abuse it. On my side, why not just focus on your one account and made it become a VIP member, so that you will have a lot of bonuses and rake back if there is and other promos that are intended for those members who have higher wagers.
There are many ways to catch people out. People often try to change up the obvious ones like IP address and account details, there are more subtle ones that can trip people up like writing style or even computer settings (browser, resolution, etc). Ultimately they don't need to figure out much, it's only at the point of cashing out they need to be sure of the final recipient. Sure people can use friends but that introduces further risks to the end user trying to multi account, like theft or failure to produce documents.