«حَتَّى لَاحُوا كَأَسْنَانِ الْمُشْطِ فِي الِاسْتِوَاءِ»
Until they appeared equal like the teeth of a comb.
Ka- introduces comparison; asnān means teeth; mushṭ means a comb. The point is not that the travellers literally shared one physique but that they showed equality and orderly fellowship.
The phrase fī al-istiwāʾ specifies the aspect of likeness. It keeps the simile precise, rather than leaving a reader to imagine irrelevant physical similarities.