We do have a KB article regarding merging people. You can find that here: https://solutions.teamdynamix.com/TDClient/KB/ArticleDet?ID=4588
It mentions that a record of the type "User" cannot be merged into another "User" type record. A non-user record can merge into another non-user record, and non-users can merge into user records. So, you might take one of those records you are wanting to merge, and downgrade the account type to a Customer. Then you should be able to complete your merge.
If you have any troubles with this, or have further questions, please let me know.