You can't do it well with your example, because it contains an odd number of characters in the input and an even number in the output, but the basic principle is:
string longWord = "aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa";
string firstPart = longWord.Substring(0, longWord.Length / 2);
string secondPart = longWord.Substring(longWord.Length / 2);