I've had an account shut down when I was using one for Southwest and the other for United.

I think e-miles did in the past. Never used e-rewards heavily since its not worth it for me. It's just a general precaution based on experience.

Some tricks I use with Gmail that might be useful in getting two accounts with the same provider (if you have Gmail):

Dots and capitalization have no meaning. So [email protected] is the same as [email protected] ; and [email protected] is the same as well. They look different but all go to the same inbox/account.

A plus sign at the end of your address is ignored by gmail; everything before the plus is used to figure out where to deliver the message and the plus and everything after is included with the message but ignored for addressing purposes. So [email protected] and [email protected] will both be delivered to [email protected]. You will be able to see the To: address so you'll know where it came from. So you can use it to see who's sold your email address, too. So if you get spam addressed to [email protected] you'll know that Delta shared your address with somebody.

The same ideas might work with other e-mail services, but I'm not aware if they do or do not work for anybody but Gmail.

Using these two, you can sometimes sign up for multiple accounts with the same service. That is if they allows dots and plus signs in the e-mail address you fill out the form with - not all do but quite a few allow them.

Are you sure that [email protected] is the same as [email protected]? I highly doubt that part and my own test didn't verify your statement.

Yes. Gmail strips out all dots from your email address.

https://support.google.com/mail/answer/10313?hl=en