You have several choices to concat two strings:
string s1 = "ABC";
string s2 = "DEF";
string r1 = s1 + s2;
string r2 = String.Concat(s1, s2);
string r3 = String.Format("{0}{1}", s1, s2);
string r4 = $"{s1}{s2}";
string r5 = new StringBuilder(s1.Length + s2.Length).Append(s1).Append(s2).ToString();
You can also copy the characters one by one in a buffer.
char[] chars = new char(s1.Length + s2.Length);
int index = 0;
for (int i = 0; i < s1.Length; i++) {
chars[index++] = s1[i];
}
for (int i = 0; i < s2.Length; i++) {
chars[index++] = s2[i];
}
string r6 = new String(chars);
[Edit] Sorry for I misread your question. You can achieve it bu only slightly modifying the code block above:
char[] chars = new char(s1.Length + s2.Length);
int index = 0;
for (int i = 0; i < s1.Length; i++) {
chars[index] = s1[i];
index += 2;
}
index = 1;
for (int i = 0; i < s2.Length; i++) {
chars[index] = s2[i];
index += 2;
}
string output = new String(chars);