Are both Apple ID's iCloud or me.com accounts? or are they 3rd party emails (i.e.; @gmail.com, etc).
If they're not iCloud specific email/apple id's you should just be able to change the email of the 2nd one and then change the email of the 1st one to the one you want.
if they're both iCloud, i don't think there is much of a way to make the switch, you can set-up them up as family sharing and download all the apps the first apple id account has.
transferring mail is pretty simple through the Mail.app on a Mac or iOS device.