Consider also that Clement never specifies whether his text had 'twelve' or 'two'
For example, after "And they were in the road going up to Jerusalem" and what follows, until "After three days he shall arise", the secret Gospel brings the following material word for word:
"And they come into Bethany. And a certain woman whose brother had died was there. And, coming, she prostrated herself before Jesus and says to him, 'Son of David, have mercy on me.' But the disciples rebuked her. And Jesus, being angered, went off with her into the garden where the tomb was, and straightway a great cry was heard from the tomb. And going near, Jesus rolled away the stone from the door of the tomb. And straightaway, going in where the youth was, he stretched forth his hand and raised him, seizing his hand. But the youth, looking upon him, loved him and began to beseech him that he might be with him. And going out of the tomb, they came into the house of the youth, for he was rich. And after six days Jesus told him what to do, and in the evening the youth comes to him, wearing a linen cloth over his naked body. And he remained with him that night, for Jesus taught him the mystery of the Kingdom of God. And thence, arising, he returned to the other side of the Jordan."
Now let's take a careful look at what Mark 10:32 does say:
They were (Ἦσαν) on their way up to Jerusalem, with Jesus leading the way, and they were astonished (ἐθαμβοῦντο), while those (οἱ) who followed (ἀκολουθοῦντες) were afraid. Again he took aside (παραλαβὼν) the Twelve (τοὺς δώδεκα) and told them what was going to happen to him.
And Matthew 20:17
Now Jesus was going up to Jerusalem. On the way, he took (παρέλαβεν) the Twelve aside (κατ’ ἰδίαν) and said to them
When you really think of it we don't pay much attention to this little story. However it is interesting. How many people were traveling on the road here to Jerusalem? The sense I've always had was that it was Jesus and the disciples. If this is true how did Jesus separate the Twelve from ... themselves? This is why the idea of Jesus taking aside two disciples - or a number smaller than 'twelve' - makes intuitive sense.
If two disciples are pulled aside and only they are told about the Passion the behavior of the disciples at the Passion makes a great deal more sense. If ten weren't aware of what was about to happen, then the chaos at the arrest scene is better explained.
“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.”
― Miguel de Cervantes Saavedra, Don Quixote