Is it permissible for one afflicted with urinary incontinence, who performs ablution for each prayer, to combine the Maghrib and Isha prayers as a "combination of advancement" (jam‘ taqdim) with a single ablution due to rain? And is his prayer valid if he prays Maghrib with the Imam, then renews his ablution and prays Isha with him thereafter?

Combining two prayers for a person with urinary incontinence is permissible according to the Hanbalis with a single ablution, and its permissibility is further enhanced if there is another excuse, such as Friday. It is also permissible according to the Malikis, as a continuous impurity does not necessitate ablution in their view.

However, your combining of prayers with the Imam without the intention of combining at the start of the first prayer is not permissible according to the majority of scholars. Ibn Taymiyyah and those who do not stipulate the intention of combining at the start of the first prayer have permitted it.