يَهْرَمُ comes from the root ه ر م and يَشِبُّ from ش ب ب. Both are present-tense verbs, but one describes ageing and the other youth or renewed vigor. Their contrast makes the teaching memorable.
In وَيَشِبُّ مِنْهُ اثْنَانِ, the word اثْنَانِ is the subject of the verb. Its dual ending indicates two things.
The practical lesson is that advancing age does not automatically remove desire. Contentment and self-discipline still require effort.