Dictionary<string, List<SomeType>> GetKeyPairValues()
{
  // get nested dictionary from this.myDict
}

Dictionary<string, List<SomeType>> GetKeyPairValues()
 {
      return dic.Values.SelectMany(d => d)
                       .GroupBy(p => p.Key)
                       .ToDictionary(g => g.Key,
                                     g => g.SelectMany(pair => pair.Value)
                                           .ToList());
 }

return myDict.Values.ToDictionary(x => x.Keys, x => x.Values);

Dictionary<string, List<SomeType>> GetKeyPairValues()
{
    foreach (var pair in dict)
    {
        yield return pair.Value;
    }
}

var dictionary = new Dictionary<string, Dictionary<string, List<int>>>();//initialize your source dictionary
var mergedDictionary = dictionary.SelectMany(d => d.Value).ToDictionary(k=>k.Key, k=>k.Value);

IDictionary<string, Dictionary<string, List<SomeType>>> dict = new Dictionary<string, Dictionary<string, List<SomeType>>>();
ICollection<Dictionary<string, List<SomeType>>> = dict.Values;