عندك سيارتان هل عندك سيارتان سيارتان : Subject (equational sentence) هل doesn't have any effect on the eraab of a sentence.

The two sentences grammatically are correct in MSA it's called dual(المُثَنَّى) the first sentence nominative(الرَّفع ) علامة رفع المثنى الالف والنون Mark dual nominative الالف and النون the second sentence accusative(ألنَّصْب) Mark dual accusative الياء and النون علامة نصب المثنى الياء والنون but if you want say that in Shami dialect you sayسيارتين and in egyptian dialect you say عربيتين

It's suppose to be هل عندك سيارتين Because It's position in the sentence is Mansub

I'm not sure wether هل affects the eraab of the sentence or not. But without هل it's عندك سيارتين.

In slang we don't focus in that so u can use both but if u want the correct one in fosha(MSA) عندك سيارتين