Building a string with only two repeating characters (different code)

Context

I would like to receive feedback on my code for the question which can be seen here.

Problem

This is another C# implementation for the a function for given parameters int a, int b - both represent characters 'A' and 'B' respectively, where the function should return a string containing both characters 'A' and 'B' occurring a times and b times respectively but neither 'A' nor 'B' repeating consecutively for more than 2 times. Both values for a and b are given in a way that a string can be build using those numbers - so eg: Foo(0,3) or Foo(1,7) shall not be invoked.

eg:

Foo(3,3) returns "BBAABA" or "AABBAB"

Foo(4,1) returns "AABAA"

Foo(3,5) returns "BAABBABB" or "BBAABBAB"

Code

static string GenerateString (int a, int b)
{

    int remainingTwiceChar = a;
    int remainingOnceChar = b;

    string twiceChar = "A";
    string onceChar = "B";

    if(b>a)
    {
        twiceChar = "B";
        onceChar = "A";
        remainingTwiceChar = b;
        remainingOnceChar = a;
    }

    int mod = (a + b) % 3;
    int div = (a + b) / 3;
    StringBuilder sb = new StringBuilder(a + b);

    for(int x = 0; x < div ; x++)
    {
        if (remainingTwiceChar >= 2 && remainingOnceChar >= 1)
        {
            sb.Append(twiceChar);
            sb.Append(twiceChar);
            sb.Append(onceChar);
            remainingTwiceChar -= 2;
            remainingOnceChar--;
        }
        else
        {
            sb.Append(twiceChar);
            sb.Append(onceChar);
            sb.Append(onceChar);
        }
    }

    for (int x = 0; x < mod; x++)
    {
        if (remainingOnceChar > 0)
        {
            sb.Append(onceChar);
            remainingOnceChar--;
        }
        if (remainingTwiceChar > 0)
        {
            sb.Append(twiceChar);
            remainingTwiceChar--;
        }
    }

    return sb.ToString();
}

Even though the implementation in the original question has a bug, my code does not have it. I attended this question a code testing platform. I guess, the code works (even it does not have the flaw that the code in the question has), however the testing platform gave only 33.33% marks to my question. Can anybody suggest me what I am doing wrong.