If you use .NET 3.5 then you can use the Union method:
Dictionary<int,> dicA = new Dictionary<int,>();
dicA.Add(1, 10);
dicA.Add(2, 20);
dicA.Add(3, 30);
Dictionary<int,> dicB = new Dictionary<int,>();
dicB.Add(3, 35);
dicB.Add(4, 40);
dicB.Add(5, 50);
var newDic = dicA.Union(dicB);
regards