قُلْ لَوْ أَنْتُمْ تَمْلِكُونَ خَزَائِنَ رَحْمَةِ رَبِّي
Say: If you possessed the treasuries of my Lord's mercy. (Al-Isrāʾ 17:100)
Conditional «لَوْ» favors a verbal construction. The detached pronoun «أَنْتُمْ» precedes the expressed imperfect verb «تَمْلِكُونَ», which identifies the sense of an omitted earlier verb.
An explanatory grammatical reconstruction is لَوْ [تَمْلِكُونَ] أَنْتُمْ تَمْلِكُونَ خَزَائِنَ رَحْمَةِ رَبِّي.
The bracketed first occurrence is not Qur'anic wording: it makes the proposed omitted verbal governor explicit. «أَنْتُمْ» is interpreted in relation to that omitted verb, while the expressed verb explains it. Treating the entire structure automatically as an ordinary subject-plus-predicate clause misses the grammatical point being argued.