The better approach here is to test the type in the method.

AddOrUpdate<TorC>( TorC entryOrEntries)
{
    if (typeof(ToC).IsAssignableTo(typeof(IEnumerable)) {
        AddOrUpdateEnumerable( entryOrEntries);
    } else {
        AddOrUpdateSingle( entryOrEntries);
    }
}

This reduces unintentional errors because the caller doesn't have to remember to specify the type when passing a value. In the other approach, if the caller forgets to specify the type then the compiler will not complain and the wrong method will be called.