Two blank lines - be consistent!
You asked the same question on some other, more specialised, sites, so I am looking through them and some others to consolidate the knowledge here for future readers on Writing.SE or others that may come across your question and want to see whether one of the three sites has additional information at a later point.
I've found that you posted your question also on stage32.com: Screenwriting : How many blank lines before scene headings (and secondary scene headings) using Amazon Storywriter? by Andy Alexander
The consensus there seems to be that you should use two blank lines because that is simply the standard. You can call it "secondary scene headings" or "sluglines" according to the answer given by Pierre Langenegger. I can't find a way to directly link to that answer, but here it is in case anyone is interested and something happens to the information on the other site:
Your question was asking for the difference in formatting between primary scene headings and secondary scene headings. My answer remains the same, they are the same element except secondary scene headings contain less detail/information.
Primary or master scene headings and secondary scene headings are both commonly referred to as sluglines. I have never used Story Sense as a reference point so I suggest you do a bit more research and you'll find they are referred to as one and the same. To be more precise, the origins of Scene Heading is purely within screenplays whereas the origins of Slugline (Slug Line) is from journalism, but regardless, your question was regarding scene headings and secondary scene headings.
Primary and secondary scene headings are the same, so if one is formatted with two blank lines you should format the other one the same way to be consistent.
I've also found your question on indietalk.com, but sadly there isn't any useful information in that thread as of writing this answer here.
You also mentioned that a lot of your confusion comes from this guide on storysense.com that mentions a difference between sluglines and scenes, where sluglines can only be used to direct attention and not to change the setting and has only one blank line as opposed to scene headers having two.
So, let's have a look at some other sources that may clear up this confusion:
scriptmag.com mentions (original emphasis):
[...] which is why I prefer the term scene heading over slug.
This shows that the author thinks they are just different names for the same thing. That source also mentions the same thing that you've been told on stage32.com: "slugline" is a term from journalism, while "scene heading" is a term from screenwriting.
Scott Myers wrote on Medium.com:
First, there is the Primary or Master Slugline (aka Scene Heading) which generally denotes the beginning of a scene and include three parts: INT or EXT, LOCATION, DAY or NIGHT.
As you can see "Slugline" and "Scene Heading" are used synonimously here.
He also has an important reminder:
Again a reminder: Screenplay format and style is not etched in stone, it is always evolving. Some writers bold primary slugs. Some underline them. Some do both. Some use secondary slugs. Others do not. Contrary to what floats around as a supposed screenwriting ‘rule’ about sluglines, you are free to approach them however you’d like as long as you are servicing the story, being clear to the reader, and making the look of your page as easy on the eye as possible.
Be consistent and everything will be fine.
Screencraft.com on the other hand mentions:
Scene headings are the general location headings that detail where we are in the script — outside or inside (EXT. or INT.) and at which LOCATION — and whether or not the reader is to envision daylight or darkness (DAY or NIGHT). It’s that simple.
Sluglines are instances within the scene description where you use CAPS to identify information that you want to call attention to.
Yeah, confusing, but the most important part I want to point out is that there is no mention of blank lines, only:
The more simple, straightforward, and consistent the scene headings are (along with the rest of the formatting elements), the better that read is going to be.
Be consistent and everything will be fine. That post also tells you that maybe you should think about when precisely you are using a scene heading. That's the most important thing. Are you switching your location or are you just pointing out that something in that scene is happening that the viewer needs to realize? For example you posted the following example in your post on stage32.com:
When I said 'secondary heading', I was referring to the 'KITCHEN' in the following example, not 'LIVING ROOM'.
INT. BOB'S HOUSE - LIVING ROOM - DAY
Stuff is happening. Bob goes through a door.
Bob opens the fridge.
Here Bob goes through the door. He changes his location. That is definitely what everyone above has referred to as a "secondary scene heading" and everyone told you to simply stick to the general formatting for "scene headings" to make it consistent and easy to write. In such a case the answer to your question is simply "2", as in: you need two blank lines, just like with your primary scene heading.
You had the terms right originally and your intuition was right, too. Go with your gut feeling, it seems to be spot on, you are simply doubting yourself too much in this case. Though it can never hurt to read a bit about official terminology and proper formatting. If you want to show that you know about the difference you should probably stick to "scene headings" as that is the screenwriting-focused term, even if others may use "slugline".