Does combining intentions in acts of worship diminish the reward?

The reward for two prayers is greater than the reward for one prayer. So, whoever prays two rak'ahs with the intention of greeting the mosque (tahiyyat al-masjid), then prays another two rak'ahs with the intention of the pre-prayer supererogatory (ratibah qabliyyah), will have a greater reward than someone who only prayed two rak'ahs for both prayers. In the Hadith: "You must prostrate much to Allah, for you will not make a single prostration to Allah except that Allah will raise you a degree by it and remove a sin from you by it."

Based on this, whoever prays two rak'ahs combining the intention of night prayer (qiyam al-layl) and the intention of a supererogatory prayer (ratibah), for example, will have a lesser reward than someone who prayed a separate prayer for each of them.