No you can't exactly merge, just transfer files between accounts using the root level "Shared" folder (or a external drive or USB) and then change it's user permissions to the other user using Get Info in the Finder and "apply to enclosed items" so all those files get changed to the second user.
What you can do, since you just migrated, is leave things alone for awhile to make sure everything is fine, just log into and use the Migrated user account.
Then later on you can log into the first account (ADmin) and turn the Migrated User to Admin (if it's not one already), log into it and delete the first account.
That would leave you with one user on the machine. (you could also transfer files to the first account, change the permissions too)
I say take your time because you might need to log into the first account (Admin) and delete the Migrated user and transfer manually or something else arises.
I run two users on my machine, one Admin that I don't use (rarely) and the regular user is General User, this way I have extra level of protection.
I wonder if
$ sudo ditto -V [old_home] [new_home]
executed from old_home would work?
The problem is that ditto preserves ownership. In SL, I assume this would be fixable by booting from the Install DVD and using Reset Password utility to reset home directory permissions. I don't know how this is accomplished in Lion.