Bismihi ta'ala
The obligatory daily prayers for all Muslims are the same times: Fajr, Dhuhr, 'Asr, Maghreb, 'Isha`.
The timing for these prayers are extremely similar for all Islamic schools of thought at as well. The point of difference as far as timing is in two aspects:
1. 'Asr,
2. 'Isha`.
The difference here is that in the Ja'fari fiqhi view, the timing for these two prayers is not as distant away from Dhuhr and Maghreb as the Sunni fiqhi view states.
In fact, for 'Asr and 'Isha`, one is able to pray them once their finish the previous salat. Yes, it is recommended to pray nawafil, and do duas, etc, between Dhuhr and 'Asr, and Maghreb and 'Isha`, but these are not obligatory to do.
So, basically, from the Shi'i perspective, it is permissible to join Dhuhr and 'Asr, and Maghreb and 'Isha`, and pray them one after the other.
In Sunni jurisprudence, this is not prohibited as well. One Sunni view is that this can be done only when travelling, with a valid excuse or in emergencies. Another Sunni view is that it can be done with a valid excuse or without.
We do know that in Sunni hadith books, there are numerous authenticated hadiths that mention the holy Prophet (s.a.w.) joined these prayers together and prayed them one after the other, without any excuse.
Therefore, what the Shi'a do in combining between these two prayers is not something heretical or far fetched, or bad. Each madhhab is entitled to have their own jurisprudential view on certain things, especially when we see that there are many Sunni scholars who also approve of it as well.
And Allah knows best.
