Salamun 'alaykum,
when it comes to Raziyyat Yawm al-Khamis, then a lot of people seem to forget to look at this [tragic] incident from a rational point of view and instead try to argue about things like whether 'Umar [and his group] accused our noble Prophet (sallallahu 'alayhi wa alihi wa sallam) to be in a delusional state or not or whether Rasulullah (sallallahu 'alayhi wa alihi wa sallam) wrote something down or not and similar to this.
If one looks however at the incident and what is certain from it, then one will see a quite obvious and logical connection to Hadith al-Thaqalayn:
When Rasulullah (sallallahu 'alayhi wa alihi wa sallam) asked for a pen and paper in order to write something down that will give protection from deviance after him, then it should be known that he (peace and blessings be upon him) had already told to the people what would protect them from deviance after him: Kitab Allah wa 'Itrati, Ahlu Bayti!
The religion was already complete and our noble Prophet (sallallahu 'alayhi wa alihi wa sallam) had already conveyed whatever was important for the people to know, so this incident was simply a reminder regarding what would protect his Umma from deviance from the correct way.
And let no one try claiming that there was no reason for a reminder: Rasulullah (sallallahu 'alayhi wa sallam) was already seeing clear signs of disobedience among some of his followers and this incidence is further proof for this! And the events that happened afterwards from Saqifa up to Jamal and Siffin and beyond clearly show that deviance and disunity happened and this clearly due to disobedience towards Rasulullah (sallallahu 'alayhi wa alihi wa sallam)!
Raziyyat Yawm al-Khamis itself is a proof for disobedience from a group from among the companions and on the top of them: 'Umar bin al-Khattab!
One can claim whatever one wants, but there is simply no justification for not accepting that a document is written down that protects from deviance! As for the claim that "the Book of Allah is sufficient", then it should be noted that the Book of Allah ta'ala was sent down upon Rasulullah (sallallahu 'alayhi wa alihi wa sallam), who was the one asking for a pen and paper! So imagine!
There is also another obvious point: The Book of Allah ta'ala is understood differently by different people, so which understanding should we take? Here the connection to Hadith al-Thaqalayn should again be obvious.