The ruling as mentioned in Shaami is derived from the Hadith of Ibn Hibbaan, narrated on the authority of A’ishah Radhiyallahu anha. However, Mufti Rasheed writes in Ahsan al-Fataawa, that Allamah Zafar Ahmad Uthmaani has quoted a Hadith also from Ibn Hibbaan, and also on the authority of A’ishah Radhiyallahu anha, which indicates that Rasoolullah separated his feet whilst in Sujood.
Thereafter, Mufti Rasheed has attempted to combine the two narrations by saying that the one mentioning the joining doesnt actually imply complete joining, it rather indicates closeness. Otherwise if a person completely joins ankles whilst in Sujood he cannot completely keep his feet straight, nor allow his toes to face the Qiblah completely. Both of these are mentioned in the Hadith of A’ishah in the view adopted by Shami. Also, in completely combining the feet whilst in Sujood, a person would be needlessly moving his feet in Salaah.
There is one explanation that the Hadith indicating separation is in accordance to the mens Sujood, also, this is closer to Khushoo’. (All from Ahsan al-Fataawa, p. 49, v. 3)
From the above-mentioned, it can be concluded that separating the ankles/feet whilst in Sujood is more prefered, as this allows the feet to stay completely straight, and the toes face the Qiblah. Also, by doing so, one avoids unnecessary movement.
I do not know the specific view and Tahqeeq (research) of my Ustaadh Mufti Umar Farook Sahib on this ruling, and at present do not have access to him either.
And Allah knows best.