Salamun Alaikum
The term al-musalli as a reference of the 'one that follows next after the foremost' applies only in the context of a
horse in a race and only because the location of the horses ‘sallan’. (Sallan - The middle part of the back of a human being or of any quadruped)
This is well known to Arabic.
This is
not the context in which the Quran uses the term. There are no horses or a race implied by the verses, nor can it be mataphorically or metonymically applied to an individual.
As you will appreciate, a root meaning must never be considered devoid of context. Words have meanings in certain contexts.
Please see a related lexicon excerpt below:
http://quransmessage.com/(7)%20Updates/links/musalli.jpgI hope that helps, God willing,
Joseph.