هَلِ الْعَنْقَاءُ مَوْجُودَةٌ؟
Does the ʿanqāʾ exist?
This illustrates simple hal, where the issue is the existence of the subject itself.
هَلْ تَبِيضُ الْعَنْقَاءُ وَتَفْرَخُ؟
Does the ʿanqāʾ lay eggs and hatch young?
This illustrates compound hal: a particular feature or action is predicated of a subject and the question asks whether that predication holds.
The distinction is between verifying existence and verifying a property of something, not between questions containing one word and questions containing several.