is it just combining Maghrib (Sunset) Prayer and `isha' (Night) Prayer on rainy days is the considered Rukhsah (concession) or they are also shortened like Salah (Prayers) shortened during travel and
recommended to be done?
Combining Maghrib and `Isha Prayers is a Rukhsah during travel, illness, and rain. It is authentically reported that the Prophet (peace be upon him) said,
"Allah likes His Rukhsah to be taken, as much as He hates to be disobeyed."
And in another narration,
"As He likes His Decisions to be followed."
It is also authentically reported that the Prophet (peace be upon him) said, regarding shortening the Salah when traveling,
"It is a Sadaqah (voluntary charity) that Allah has granted you, so accept His Sadaqah."
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.