Interleaving two strings
An if in a tight loop is usually a performance killer. At least worth trying is something along the lines of

    len = min(args[0].Length, args[1].Length);
    for (j = 0; j &lt; len; ++j) {
        Console.Write(args[0][j]);
        Console.Write(args[1][j]);
    }
    WriteRemainingsOfLongerString();


As mentioned in the comments, it could also be more performant to build a string and Console.Write it at once.