Sheikh Khâlid al-Mushaykih, professor at al-Imâm University, al-Qasîm branch
Under certain circumstances, it is permissible for the worshipper to combine Zuhr with `Asr at the time of either of those two prayers. Likewise, under the same circumstances, the worshipper may offer Maghrib with `Ishâ’ together at the time of either of those two prayers.
A condition for the validity of offering two prayers together is to avoid any separation between them. This means that praying them one after the other, in immediate succession, is a condition for the valid combination of the two prayers.
The evidence for this is the hadîth that the Prophet (peace be upon him) prayed the two midday prayers (meaning Zuhr and `Asr) and the two evening prayers (meaning maghrib and `Ishâ’) as eight units and seven units, respectively. [Sahîh al-Bukhârî (529)]
However, it is alright to separate between the two prayers with a small interval of time, as mentioned in the Shâfi`î school of Islamic jurisprudence. A small interval of time is of no legal consequence.
However, no voluntary prayer can be performed between the two combined prayers, neither during travel nor during residence, since that would constitute a tangible separation between the two prayers.
And Allah knows best.
Source: Islam Today