If both profiles are public and you manage or collaborate with the manager of one, you can merge them.
If one profile is private - and you're the manager - the best option is to make it public and then merge. If you are not the manager and not in the family group of the manager, contact the manager to make it public so it can be merged.
If you want to email me on Geni with the URL (Internet address) of the proposed merge, I'll take a look and see if I can advise you more specifically. Usually it's an issue of one being private or the person doing the merge not collaborating with the manager of the other profile.