Is it permissible to combine two intentions in supererogatory prayers or fasting?

It is permissible to do so, as intentions are said by scholars to be a sort of trade. Suppose a Muslim fasts on Monday or Thursday which are of the days that Prophet Muhammad (Peace be upon him) superfluously prefers to fast as he (Prophet Mohammad) said in Sahih " The deeds of people would be presented on Mondays and Thursdays, and I like to have my deeds presented to Allah while I am fasting." Related by At-Termizey and said it is a good Hadith (saying of the Prophet) -- and it happens that the day (Monday or Thursday) was coincidentally one of the white days { 13^th , 14^th , and 15^th of every Hijri (AH) Month} that Prophet Mohammad superfluously prefers to fast, it is permissible then for a Muslim to combine both intentions for fasting that day. Another example that is permissible is when a Muslim gets into a mosque and performs a two Rak`ah (individual unit of Muslim prayer) prayer as a greeting for the mosque and intends them coincidentally to be as Istikharah (a prayer performed to ask for Allah`s guidance and inspiration on a certain issue.) The rule on that issue depends on Allah`s Apostle saying " Deeds (their correctness and rewards) depend upon intentions, and every person gets but what he has intended."