We know that Sunnis believe that performing noon and afternoon prayers at one time, and Evening and night prayers together is wrong and invalid.
We must perform each prayer in its own time and separate the time of noon and afternoon prayers and also put a distance between prayers of evening and night.
Dr. Tijani Samavi says: I was a Sunni and performed my prayers according to this law. I believed it is invalid to perform them two by two and together.
When I arrived in Najaf Ashraf, My friend helped me to meet Ayatollah Shahid Muhammad Baqir Sadr.
At noon, Mr Sadr set off to mosque and all the present people including me followed him to perform the prayer. I noticed that Mr Sadr started the afternoon prayer in a short while after noon prayer.
I wasn’t in a condition to leave the queue. So it was my first time that I performed the two prayers together. However I was worried if my prayer was correct or not?
That day I was a guest of Shahid Sadr. I got an opportunity of presenting myself to him. I asked: “Is it correct if a Muslim performs two daily prayers at once in the time of emergency?”
Yes, it’s correct. It is possible to perform two daily prayers (noon and afternoon, Evening and night) in a row without being in an emergency situation.
I asked: what is your reason?
“Because Rasoul-Allah (pbuh) prayed this way in Medina while there was no situation of fear or rain or any emergency and performed noon and afternoon prayers together and also evening and night prayers together. His highness wanted to remove difficulty from his nation. Be Hamd-Allah this action has been further proved by our Imams and Ahl al-Bayt (peace be upon them) to us.
However, it is also proved to you Ahl al-Sunat through Sunah.”
I wondered how it is proved to us; despite I hadn’t heard about it before and had not seen any Sunni who did the same. Otherwise they said: if a prayer is performed even one minute before Azan it is invalid, so if anybody performs afternoon prayer immediately after noon prayer or performs his night prayer immediately after evening prayer, we believe it’s invalid and wrong.
Mr Sadr realized from my face that I am wondering how it is allowed to perform daily prayers together.
He beckoned a student present there; he went and came back with two books for me. I saw they are Sahih Muslim and Sahih Bukhari.
Mr Sadr told that student to show me the narrations about the gathering of the two prayers in those books.
I read there that Rasool-Allah (pbuh) had performed the prayers of noon and afternoon at one time and the prayers of evening and night together in a normal situation, not in danger, rain or any other emergency.
I found a complete section about this issue in Sahih Muslim.
I was bewildered, My Lord, what do I see here? I doubted if the versions they have are distorted and they aren’t originals. I decided to follow this issue when I returned to Tunis and look for it in our books there.
At this moment Shahid Sadr asked me:
“This was the reason, Now what do you think?”
I said: “you are righteous and honest…” I thanked him but I wasn’t persuaded.
Until I returned to my home country Tunis, and in the first opportunity I got Sahih Muslim and Sahih Bukhari…and investigated them carefully.
I was completely persuaded that performing noon and afternoon prayer together and evening and night prayers together-- even not in an emergency situation is correct, since our Prophet Muhammad (pbuh) had done this way.
I saw that Imam Muslim in his Sahih  wrote in the section of “together performance of two prayers in trip” a narration by Ibn-Abbas that Prophet (pbuh) prayed noon and afternoon prayers together, and evening and night prayers together.
He also narrates that: Prophet performed the prayers of noon and afternoon together in Medina and so did prayers of Evening and night. People asked from Ibn Abbas why Prophet did this?
He said: «كي لا يحرج امته؛ not to put the nation into difficulty. 
And also in Sahih Bukhari  I saw and read in the section “Vaght al-Maghrib” (the time of evening), it was quoted from Ibn-Abbas that our Prophet performed 7 Rak’ats (Maghrib and Isha) together, and 8 Rak’ats (Zuhr and Asr) together.
In Musnad Ahmad  I saw a similar narration and in the book al-Mouta Imam Malik I saw another narration by Ibn-Abbas:
«صلي رسول الله(صلي الله عليه و آله):
الظهر و العصر جميعا و المغرب و العشاء جميعا في غير خوف و لا سفر؛
Prophet Muhammad (pbuh) performed the prayers of Zuhr and Asr together and also the prayers of Maghrib and Isha; while he was not in trip or danger.
So when this issue is crystal clear for us, why Ahl al-Sunat makes this issue  a big problem of Shi’a? Regardless of the fact that it has been authenticated in their own original books! 
 Sahih Muslim: v2, p151, (section al-Jam Bain al-Salatayn fi al-Hazar)
 ibid, p 152.
 Mosnad Ahmad, v1 p 221
 ibid, p140
 al-Moota al-Imam Malik, v1, p161
 it refers to prayers of Zuhr and Asr, Maghrib and Isha prayed together.
 la kan ma al-Sadeghin: Tijani Samavi, Bayrut, pp21-214 (abridged).