I wrote this algorithm

Input data

• weekCount : total number of weeks to be examined
• skipWeeks : number of weeks to skip / skip forward. In your example where "The occurrence will happen every 2 weeks", skipWeeks would be 1. That is, the algorithm will examine a week, advance 1 week, examine the week, advance 1 week, etc.
• daysOfWeek : days of the week (ex: new[] {DayOfWeek.Monday, DayOfWeek.Thursday} )
• (optional) startDate : day to start the search. By default this is the current day.

public IEnumerable<DateTime> AgendarAlertas(int weekCount, int skipWeeks, IEnumerable<DayOfWeek> daysOfWeek )
{
    return AgendarAlertas(weekCount, skipWeeks, daysOfWeek, DateTime.Today);
}

public IEnumerable<DateTime> AgendarAlertas(int weekCount, int skipWeeks, IEnumerable<DayOfWeek> daysOfWeek , DateTime startDate)
{
    var dates = new List<DateTime>();

    Calendar calendar = new GregorianCalendar();
    DateTime firstDayOfWeek = DateTime.Today.StartOfWeek(DayOfWeek.Sunday);

    for (int currentWeek = 0; currentWeek < weekCount; currentWeek++)
    {
        //calcular dias da semana
        dates.AddRange(
            daysOfWeek.Select(dayOfWeek =>
                                firstDayOfWeek.NextDayOfWeek(dayOfWeek)));

        //avancar "skipWeeks"
        firstDayOfWeek = calendar.AddWeeks(firstDayOfWeek, skipWeeks + 1);
        currentWeek += skipWeeks;
    }

    //filtrar dias passados
    return dates.SkipWhile(date => date < startDate);
}

2 extension methods

public static class DateTimeExtensions
{
    public static DateTime NextDayOfWeek(this DateTime dt, DayOfWeek day)
    {
        int diff = day - dt.DayOfWeek;

        //se o dia ja passou, calcular o da proxima semana
        if (diff < 0)
            diff += 7;

        return dt.AddDays(diff).Date;
    }

    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        int diff = dt.DayOfWeek - startOfWeek;
        if (diff < 0)
            diff += 7;

        return dt.AddDays(-1 * diff).Date;
    }
}

Result:

AgendarAlertas(11, 1, new[] {DayOfWeek.Thursday, DayOfWeek.Friday});

06/02/2014 00:00:00 //proxima quinta-feira
07/02/2014 00:00:00 //proxima sexta-feira
20/02/2014 00:00:00 //proxima quinta-feira + 2 semanas
21/02/2014 00:00:00 //proxima sexta-feira  + 2 semanas
06/03/2014 00:00:00 //proxima quinta-feira + 4 semanas
07/03/2014 00:00:00 //proxima sexta-feira  + 4 semanas
20/03/2014 00:00:00 //proxima quinta-feira + 6 semanas
21/03/2014 00:00:00 //proxima sexta-feira  + 6 semanas
03/04/2014 00:00:00 //proxima quinta-feira + 8 semanas
04/04/2014 00:00:00 //proxima sexta-feira  + 8 semanas
17/04/2014 00:00:00 //proxima quinta-feira + 10 semanas
18/04/2014 00:00:00 //proxima sexta-feira  + 10 semanas

Option 1:

public static void obterDiasComIntervaloSemanal(DateTime data, int intervalo, int total_semanas, DayOfWeek[] dias)
{
    int semana_atual = 1;    
    while (semana_atual <= total_semanas)
    {
        if (dias.Contains(data.DayOfWeek) && semana_atual % intervalo == 0)
            Console.WriteLine(data.ToString("dd/MM/yyyy")); // Aqui você decide o quer fazer se quiser adicionar numa lista de datas a data atual...
        if (data.DayOfWeek.Equals(DayOfWeek.Sunday))
            semana_atual++;
        data = data.AddDays(1);
    }
}

And to use:

static void Main(string[] args)
{
    DayOfWeek[] dias = {
                       DayOfWeek.Monday,
                       DayOfWeek.Thursday
                   };
    obterDiasComIntervaloSemanal(new DateTime(2014, 02, 01), 2, 10, dias);
}

Result:

Option 2:

public static DateTime getDateInterval(int year, int month, int week, DayOfWeek day)
{
    int current_week = 1;
    var days = DateTime.DaysInMonth(year, month);

    DateTime date = new DateTime(year, month, 1);

    for (int i = 1; i <= days; i++)
    {
        if (date.DayOfWeek == day && current_week == week)
            break;
        if (date.DayOfWeek.Equals(DayOfWeek.Sunday))
            current_week++;
        date = date.AddDays(1);
    }

    return date;
}

To use:

static void Main(string[] args)
{
                           // year month week   day of week
    var data1 = getDateInterval(2014, 2, 2, DayOfWeek.Monday); // 03/02/2014
    var data2 = getDateInterval(2014, 2, 2, DayOfWeek.Thursday); // 06/02/2014
    var data3 = getDateInterval(2014, 2, 4, DayOfWeek.Monday); // 17/02/2014
}

Solving your problem in the simplest way: one for iterating directly over dates.

It looks like this:

var date = new DateTime(2014, 2, 4);
var intervaloSemanas = 2;
var diasDaSemana = new DayOfWeek[] { DayOfWeek.Monday, DayOfWeek.Thursday, };

var dataDeStop = date.AddDays(80);
var incremento = TimeSpan.FromDays(7 * intervaloSemanas);

for (var dt = date.AddDays(-(int)date.DayOfWeek);
     dt <= dataDeStop;
     dt += incremento)
{
    foreach (var dayOfWeek in diasDaSemana)
        Console.WriteLine("{0:yyyy'-'MM'-'dd}", dt.AddDays((int)dayOfWeek));
}

Output:

This code is also very fast because it goes straight on the dates you want.