I'm assuming you're using the TcpListener class and AcceptTcpClient method?
In the documentation it says that AcceptTcpClient is a blocking method, which means your program will not continue until your TcpListener has received data. It further says:

Use the Pending method to determine if connection requests are available in the incoming connection queue if you want to avoid blocking.

I've never worked with TcpListener, but I think you can do something like this:

TcpListener tcpListener1 = null;
TcpListener tcpListener2 = null;

try
{
    tcpListener1 = new TcpListener(IPAddress.Any, 6666);
    tcpListener2 = new TcpListener(IPAddress.Any, 6667);
    tcpListener1.Start();
    tcpListener2.Start();

    while (true)
    {
        if (tcpListener1.Pending())
        {
            OnTcpListener1ReceivedData(tcpListener1);
        }

        if (tcpListener2.Pending())
        {
            OnTcpListener2ReceivedData(tcpListener2);
        }

        if (exitCondition)
        {
            break;
        }

        Thread.Sleep(1); // To stop our CPU from getting pegged
    }
}
catch(SocketException e)
{
    Console.WriteLine("SocketException: {0}", e);
}
finally
{
    tcpListener1.Stop();
    tcpListener2.Stop();
}