Customize Sluglines in Scriptwriting?

Is there any way to customize the sluglines in iOS or Mac OSX? Specifically, to make them bold and to have 2 line spaces?

Indeed, both of those options are a possibility. You’re looking for the Format ▸ Scriptwriting ▸ Script Settings… menu command. Documentation for the panel is found in §19.7, Creating Your Own Script Formats, but you’ll be most interested in §19.7.2, where the Font (bold) and Paragraph (spacing) tabs are discussed.