Easiest way: pick a character that can't be in eitehr string:
string a = "foo1";
string b = "foo2";
string combined = string.Join("|", a, b);
To separate them again, just use Split:
string[] parts = combined.Split('|');
string a = parts[0];
string b = parts[1];
But if you are doing this to store the information in a file or database, there are probably better things you can do instead of messing with string concatenation at all!