Is it a condition not to have a short break between the two combined prayers (Maghrib and Isha or Dhuhr and Asr), and is there any evidence for the necessity of the immediate succession of the two prayers, i.e., performing the iqama directly between them?

The fundamental principle for combining two prayers is continuity (muwalat), based on the practice of the Prophet, peace and blessings be upon him. If the combination occurs during the time of the first prayer, then continuity between them is essential. If a significant amount of time separates them, the combination becomes invalid according to the Maliki school. However, if the combination occurs during the time of the second prayer, the correct view is that a separation between them, even if lengthy, does not invalidate the combination.