There is no current way to force passengers through a specific set of security check points, when all security check points lead to the same secure area.
To achieve what you want, you do need to create separate secure zones. I know it seems frustrating/difficult at first when you have your ideal layout in mind, but once you get it right, it is pretty seamless and flows very nicely. Plus, smaller secure zones work best in the current game set up, because, as you have experienced, the passengers have somewhat of a mind of their own.
You are correct that you “need” to have it one secure zone when you have all your stands next to each other, like in your screenshot, because the stand’s own zoning join the terminal’s zones even if the secure zone is appear to be separated in the terminal. The way I have learned to achieve this is to ensure there is a strip of unsecure squares that runs between my two security check points all the way through, dividing my two zones, until it hits a wall that has grass on the other side. The trick to remember, is one tiny, open square of secure zone can bridge two huge “terminals.” i.e. when the doorway/jetway to a stand is created, it bridges the stand’s secure zone to the terminals, and if that stand is directly bordering another, it absorbs it into its own zoning along with whatever else was securely attached to it, and so on. It is a devilish cascading effect. Then you need to save and reload if to see if you successfully separated them.
There has been plenty of discussion is previous threads with requests to allow the zoning you showed us, but allow players to assign specific security check points to specific stands all within the same secure zone. Don’t quote me on this, but I believe a revamp of the whole security checkpoint and process is one of the next “bigger” items to get on the devs’ plates after they satiate all us impatient players who want multi floor, fixed weather, catering, cleaning, etc. Basically, there is a functioning system that is due to get a major overhaul. It would be a waste to do a “quick” fix that can easily be remedied with some reconfiguring of your airport. I’ve learned this the difficult way that there is a method to the madness of the development of this game, and it does work really well. I have found that the beauty of the challenge of the game is to get it to work out for the builds I want.