What is the criterion for the overlap of acts of worship and the overlap of intentions?

The questioner : A question.

The Shaykh : Regarding the oath, the discussion has ended. Men, all the guests have a right, there is no delay for the one who asked... so-and-so, meaning we promised him. Take, take, the one who has the right to record.

The questioner : Peace be upon you, Shaykh (may Allah preserve him). Regarding the overlap of acts of worship...

The Shaykh : Regarding what?

The questioner : ...the overlap of acts of worship, such as the Sunnah of 'Isha' and the prayer for guidance (istikharah), for example, or fasting on Mondays and Thursdays along with the six days of Shawwal. How do we combine the intentions, and what is the criterion, O Shaykh, so that we know that this does not overlap with that, and this with that?

The Shaykh : If the intention is the act of overlap, and if the intention is that the same act of worship does not overlap, for example, the Prophet (peace and blessings be upon him and his family) said: So a man entered and prayed two rak'ahs of the regular voluntary prayer (rawatib) of Fajr. Now, did he sit before he prayed the two rak'ahs, or did he pray the two rak'ahs?
The student: He prayed.

The Shaykh : He prayed two rak'ahs, and the intention was to pray two rak'ahs, and that has been achieved. Okay, good. In the Sunnah of Zuhr, there are two rak'ahs and two rak'ahs before the prayer. If he intends to make the two rak'ahs count for the four, is that sufficient? It is not sufficient. The intention is to perform the same four rak'ahs. Regarding the prayer for guidance (istikharah), the scholars (may Allah have mercy on them) differed regarding the saying of the Prophet (peace and blessings be upon him and his family): If he prays the regular voluntary prayer (rawatib) and then performs istikharah, does he achieve the intended purpose, or must there be a separate prayer for istikharah? Whoever said that his saying: includes all voluntary prayers, even the regular voluntary prayers (rawatib), even the greeting of the mosque, even the Sunnah of ablution, said that it is sufficient. He can perform istikharah after the regular voluntary prayer, there is no obstacle. And whoever said that this from the Prophet (peace and blessings be upon him and his family) indicates that there must be a separate prayer for istikharah, and this is closer to the truth in my opinion, said that it is not sufficient unless he prays a specific prayer for istikharah. Yes. As for the issue of Mondays and Thursdays, if a person fasts on Mondays and Thursdays for three weeks, it is true that he has fasted six days of Shawwal, and he will receive the reward of fasting on Mondays and Thursdays. But in the six days of Shawwal, it is better to continue fasting consecutively than to specifically target Mondays and Thursdays, meaning that you fast immediately after the festival and complete all six days.

The Shaykh : With the guests
The student: ...