I've never done it large scale, but I have about 3 accounts at
Paddy Power for whatever reason. I think its mainly because one day I wont be able to remember my login details and cant be bothered using the customer service dept at that particular time - quicker to open a new account
I made no effort to hide it - all usernames are pretty much identical as are the addresses, I might have changed a letter or two just incase they have some sort of matching software on names and addresses.
And I got the new account bonuses on all of them, no problems. Got a few at
Bet365 too - again, no problems.