How to bind data that is saved to a file with other data that is saved to another file c#

Why don't you create file per user ? You can just change "SuggestingDifficulty" functions to get the username and use it as file name ?

public static UserDifficulty SuggestingDifficulty(string userName)
{
    ToFile objnew = SaveToFile.DeserializeLastTest($"{userName}.txt");
    Console.WriteLine($"Last time you did the test on {objnew.UserDifficulty} level and got {objnew.TotalScore}/{objnew.NumberOfQuestions}");
} 

and

public static void SerializeLastTest(int numberOfQuestions, int totalScore, UserDifficulty userDifficulty,string userName)
{
    ToFile obj = new ToFile(numberOfQuestions, userDifficulty, totalScore);
    IFormatter formatter = new BinaryFormatter();
    Stream stream = new FileStream("${userName}.txt", FileMode.Create, FileAccess.Write);
    formatter.Serialize(stream, obj);
    stream.Close();
}

public static ToFile DeserializeLastTest(string userName)
{
    Stream stream = new FileStream($"{userName}.txt", FileMode.Open, FileAccess.Read);
    IFormatter formatter = new BinaryFormatter();
    ToFile objnew = (ToFile)formatter.Deserialize(stream);
    stream.Close();
    return objnew;
}