How can I switch from one email user to another?

I don't know if this answers your question, but select whichever email account that you want to use on your iPad. I have four email accounts on mine. Of course, you must have all of your email accounts set up in order to use the mail app, or you can use a web browser and access any email accounts that you have on the web.

If that's not what you were asking about, - and I am guessing that it wasn't - why don't you clarify a bit?

You have to select between the two accounts in the mail app.

Say for instance that you are reading an email in your XYZ email account. Tap the button in the upper left corner of the mail app to back out all the way to where you can see all of the email accounts.... Tap InBox, then tap XYZ email Account and that will take you back to the main MailBoxes window.

Is that what you want to know? There is no way to simply "toggle" between the two accounts.

Do both email accounts in Settings > Mail, Contacts, Calendars have Mail set 'on' i.e. are they both 'active' on the iPad ? If they are then you should be able to go up another level in the Mail app and be able to select/navigate into the other account.