The 'Hanbali'...
The 'Hanbali', Maliki and Shafa-ee say: The joining of the noon and afternoon prayer or the 'Maghrib' and Isha one, at a special situation in addition to the two mentioned cases and whilst traveling becomes permissible. Some of these groups consider it permissible to join the prayers when necessary such as sickness or fear of an enemy at lurk. The Shia believe that each of the noon as well as the afternoon, and the 'Maghrib' and the Isha prayers, have a 'special time' and a 'joined time'.
The special time of the noon prayer begins from mid-day till the time when the four units of prayers have been recited and this is a limited period in which only the noon prayer can be performed. The special time for the afternoon prayer is the time starting from that particular moment until sunset, only considering up to the period of performance of the afternoon prayer.
The joint time that is between the two prayers of the noon and afternoon, starting from the end of the special time noon prayer to the commencing of the special time of the afternoon prayer. The statement of the Shias to this regard is that between any time of the joined period, one can perform both the noon and the afternoon prayer jointly and without any interruption.
However the Sunnis believe that from the beginning of the legal noon (mid-day) up to the time when the shadow of everything is equal to its length, is the specific time for the noon prayer and during that time, one should not pray the afternoon prayer, but after this time has elapsed to the time of sunset, it becomes exclusive for the afternoon prayer and one cannot pray the noon prayer during this interval.
The specific time for the 'Maghrib'prayer is from the beginning of the legal 'Maghrib' time until the time that the three units can be performed. The special time for Isha prayer is the time starting from that particular moment up to the time of being able to perform the Isha prayer, from midnight. The joined time for the two prayers of 'Maghrib' and Isha is from the end of the special time of 'Maghrib' prayer and prolongs up to the beginning of the special time of Isha prayer.