One thing that many people are confused about is dual delivery (aka split delivery). Simply put, this means that emails will be delivered to two email systems simultaneously. This is often used to keep a new and a legacy system live in parallel. It is also frequently used during email migrations to ensure that no downtime occurs.
The concept of dual delivery isn't very hard to wrap your head around, but how does it work? When reading forums, some people are confused about this. Some people think that it is sufficient to have two servers with the same priority level in the MX-record. This is not the case. Simply having two MX-records with the same priority will load balance between the two, but not deliver to both.

In order to set up dual delivery, you need to have a primary email server relay the messages to the secondary server. How to configure this varies a lot between different email servers and solutions. For instance, the instructions for setting this up with Google Apps can be found here.
Now that you understand how this works and how it is configured, it may be a good time to discuss the pros and cons. The obvious benefit with using dual delivery is redundancy. You get a second copy of all incoming messages. The drawback however, is that this only solves the issue of inbound emails. It does not solve the issue of organizing the mailboxes. If a user is using the secondary server, his folder-structure, organization of messages, and read/unread status will not get propagated between the two servers. It is possible with certain setups to accomplish this too, but that's not within the scope of dual delivery per se.
Please comment below if you have any questions, and as always, check out YippieMove for all your email migration needs.
Want to learn more about email and email systems? Learn more in The Email School.