Praise be to Allah.
When joining prayers during the time of the first of the two prayers, they should be prayed one after the other. It is OK to have what is regarded as a short break between them, as it was proven that the Prophet (peace and blessings of Allaah be upon him) did that. And he (peace and blessings of Allaah be upon him) said: “Pray as you have seen me praying.” (Narrated by al-Bukhaari, Kitaab al-Adhaan, 631). The correct view is that the intention (niyyah) [i.e., having the intention to combine the prayers before starting the first prayer] is not a condition. But with regard to joining prayers during the time of the second of the two prayers, there is more leeway, because the second prayer is being performed at its right time. But it is better to pray them one after the other, following the example of the Prophet (peace and blessings of Allaah be upon him). And Allaah is the Source of strength.