What you ask is quite doable.
On one device, add the account you wish to get Messages from in the messages settings. It will ask for the user name and password. If that device is a phone, you will have to register that email to be a receiver for iMessage in addition to the cellular phone number. If the device is an iPod touch or iPad you can only receive iMessages to an email address / Apple ID.
Now for the email, go to the Mail, Contacts and Calendars settings and add both accounts.
There you go - email comes in on two accounts and messages come in on one. Repeat these steps as needed on your second, third and other iOS devices. When you have completed the registration, iMessages should propagate to all devices that are enabled for a particular addressee.
The only potential problem would be if you had an iPhone and tried to add an email to be a secondary receive address as well as the phone number and that email was already tied to another different cellular number. In that case, you would have to detach that email from that other number before attaching it to the new number.