Do you deny that the Shia and the Sunni follow different customs ( aka Sunnah )?
I think I've made two posts that talk about the differences in what they hold about the Sunnah. That should satisfy your question.
Editing to add:
This may help in the question.
https://www.al-islam.org/shiite-encyclopedia-ahlul-bayt-dilp-team/major-difference-between-shia-and-sunni
بِسْمِ اللَّـهِ الرَّحْمَـٰنِ الرَّحِيمِ
All the Muslims agree that Allah is One, Muhammad (S) is His last Prophet, the Qur’an is His last Book for mankind, and that one day
Allah will resurrect all human beings, and they will be questioned about their beliefs and actions. There are, however, disagreements between the two schools in the following two areas:
1. The Caliphate (successorship/leadership) which the Shi’a believe is the right of the Imams of Ahlul-Bayt.
2. The Islamic rule when there is no clear Qur’anic statement, nor is there a Hadith upon which Muslim schools have agreed.
The second issue has root into the first one. The Shi’a bound themselves to refer to Ahlul-Bayt for deriving the Sunnah of Prophet (S). They do this in conformity with the order of Prophet reported in the authentic Sunni and Shi’i collections of traditions beside what the Qur’an attests to their perfect purity.
The disagreement about the caliphate should not be a source of division between the two schools. Muslims agree that the caliphate of Abu Bakr came through election by a limited number of people and was a surprise for all other companions. By limited number, I mean, the majority of the prominent companions of prophet had no knowledge of this election. ‘Ali, Ibn Abbas,
Uthman, Talha, Zubair, Sa’d Ibn Abi Waqqas, Salman al-Farsi, Abu Dharr,
Ammar Ibn Yasir, Miqdad, Abdurrahman Ibn Owf were among those who were not consulted nor even informed of. Even Umar confessed to the fact that the election of Abu Bakr was without consultation of Muslims. (See sahih al-
Bukhari, Arabic-English, Tradition 8.817)
On the other hand, election implies choice and freedom, and that every
Muslim has the right to elect the nominee. Whoever refuses to elect him does not oppose God or His Messenger because neither God nor His Messenger appointed the nominated person by people.
Election, by its nature, does not compel any Muslim to elect a specific nominee. Otherwise, the election would be coercion. This means that the election would lose its own nature and it would be a dictatorial operation.
It is well known that the Prophet said: "There is no validity for any allegiance given by force."
Imam ‘Ali refused to give his allegiance to Abu Bakr for six months. He gave his allegiance to Abu Bakr only after the martyrdom of his wife [Continues @ link:
The Major Difference Between the Shi’a and the Sunni )
Conspiracy Against Imam Ali (as)