I speculate that "sabʿan mina l-mathānī" in 15:87 could be referring to “l-samāwāti” two verses before it in 15:85 seeing as l-samawati appears with sab’a (the number 7) in 2:29, 17:44, 23:86, 41:12, 65:12, 67:3 and 71:15
After reading Farouk's Blog Post on Quranology Blog about 15:87, where "folding" was mentioned, I investigated the word folding and found “naṭwī” which is also linked to “l-samāa” in 21:104 - this is also to do with folding, as per the Quranic example of “folding a book". If we try out the example ourselves and take an open book from flat to closing it, this makes a “V” shape. When I look at 20:12 now – “bil-wādi l-muqadasi ṭuwan” - I get a mental image of a V shaped valley.
39:67 also mentions the “wal-samāwātu maṭwiyyātun” (this links the samawati from 15:85 with the "folding" concept)
Spookily, I noticed the number 7 in arabic digits is like a V shape too.
(11:5) yathnūna is from the same root as mathani (in 15:87) and is referring to the ṣudūrahum this time.
So yathnuna / mathani and natwi / tuwan / matwiyyatun are 2 different roots, both implying something to do with folding. Seeing as the mathani root seem to connote "pairs" or the number 2, I am leaning towards "doubling up" for 11:5 in reference to the sudurahum.
Chapter 11. This doubling up of the sudurahum in 11:5 seems to be an attribute of the people who are mentioned to be ones who are hiding, covering and concealing. Then come several mini stories of the Messengers/Prophets. After those stories we see a "fa-is'taqim" which to me is summarising the previous passages of the examples of the behaviour of the Messengers/Prophets throughout this Surah, and gives me a mental picture of the contrast between standing upright, being assertive and determined and having courage, as opposed to cowardice, hiding away, concealing, being deceitful.
The Surah prepares to conclude with "iʿ'malūʿalā makānatikum" in 11:121 which to me is asking us to reflect on our own "posture" or position and do our deeds accordingly; the question is, are we "doubled up" or "standing upright"?
The chapter then finally concludes with a reminder that God is not unaware of anything we do.
Getting back to what started this chain of thoughts off, if mathayni is to do with 2 and doubling or pairs, then maybe it's not just "samawati" that is referred to, seeing as usually "samawati" is mentioned along with "wal-ardi"