When you have multiple applications assigned to 1 resource, then it is also possible that more than resource are handling the same application (assuming the support effort for each application is 1 person-month for a month). In such cases, they work in a primary-secondary model i.e. 1 resource is primary and the other/s secondary.
To ensure that the secondary resource is trained and always up to the required knowledge level is a challenge. Generally it is seen that this is not implemented well and if the primary is not available for whatever reasons, the secondary has a tough challenge ensuring the level of support demanded by the service level agreement.
A good cross-training plan within the team would eliminate such issues.