@ K1W1: I only have three. Though I do have GMail set up twice, once as Exchange and once as IMAP. Does that count?
@ OP: Once you have a second email account set up you'll notice that when you tap the MailBoxes button at the top left of the email list that you have some new options. One is the All Inboxes, which will let you view all your new email at once. It doesn't merge them. It is just a convenience. You can also choose to view them individually, or choose from the account list below to view folder other than the inboxes.
Be sure to go to Settings > Mail, Contacts, Calendars and select a default account. When you send mail from other apps, or when you are viewing the All Inbox, this is the account you will send from by, well, default. You can change it when editing the email, but if you are like me you tend to forget that; so pick the email you want your replies to come to, at least most of the time.
When forwarding or replying to an email, it will always use the same account it came too, so no worries there.
