how do I merge two logins?

Ok, duplicate users; not duplicate data, and you don't easily merge data if at all. Users are separated intentionally to help prevent corruption and or data "theft" for lack of a better term. User B data is non accessible to use A unless you login as B and give control to user A.

Depending on how much time you want to invest, you could copy folder by folder the original data to an external drive. If you then copy and paste data from the external to new Mac, on an individual folder basis, complete into less complete, you'll be asked if you want to overwrite duplicate files and folders, select to skip those and only the "new" files will be copied over. Or you can do a slam dunk and copy over everything replacing the duplicates ... your choice. But one user needs to go.

Or, remove one of the users completely, data and home folder, and migrate the data all over again selecting to overwrite the existing user. This assumes you still have the old dataset on the old computer and you trust the migration process.

I assume the data you migrated is the data you want to keep and you used either Setup or Migration Assistant.

If they are true duplicates you don't need to merge, just remove one along with its data and home folder.

When you migrated the data you must have selected the option to not replace the current user.

If it doesn't work to your satisfaction, after removing one user, re-migrate the data and select the option that replaces the user with the known good data from your backup or old computer.