This could be done with some core modifications (ie. if you hide the
username you'll have to assign the email to the username on save, and
if you hide the email you'll have to save the username to the email
field (I think this would probably be the easiest to ensure no ripplie
on effects).
However, isn't it easier just to tell your users to enter their email
address in both field - then you don't have to do any modifications,
and what if the user wants to change their email address? Usernames
cannot be changed in the current version of DNN (and I don't know if
they ever will be able to), so then the user is stuck with that email
address for life...
Just some thoughts...