There can be many kinds of Guardians. A sole Guardian is a person who you nominate in your Last Will to take care of your minor children until they legally become adults. Co-Guardians refers to nominating two individuals instead. Note also that co-Guardians need to almost always live in the same city or town. Additionally, most courts or governments prefer to appoint married couples as Guardians.
An alternate sole Guardian is a single individual taking over the guardianship of your children. This happens if the original Guardian(s) cannot or will not serve according to your nomination. If two people take over instead, they are alternate co-Guardians. Again, most courts have a clear preference for married couples regarding guardianship.