The only way two belts could be affixed to the railing would be to use the buckle of one belt in the holes of the other to make one huge loop. It would require the killer to loop this larger belt over the railing. There are two ways to do it. First, using an infinity (figure 8) design looped over the top of the railing then placing the head of each deceased person in the loops. Being already dead the weight of each person would keep the bodies partially erect. The second way would be to loop one belt over the railing then buckle it into the other's holes. This way the belt can't be removed without unbuckling it. Then you'd have to pull it forward and make two individual loops and have the deceased person or perhaps an unconscious person help asphyxiate themselves by their own body weight.
The safety requirements for having railings around a pool like this may have changed over the years. When I look at the pictures of the pool area it looks like the railings are around 36 to 42 inches high, probably closer to 36 inches. (One of the most unappealing looking indoor pools I've ever seen, btw). They don't look like they would keep little kids safe since there are no metal uprights.
Apparently the belts were Barry's. How big would you say Barry's waist was? Maybe 38 or 40 inches? A belt for someone that size would be anywhere between 41 and 42 inches long. Here's a link to that info.
https://www.sizecharter.com/clothing-fit-and-measurement/belt-size-charts
If the railings are 36 inches high that means a belt looped around the railing would have a drop of anywhere between 19 to 20 inches considering the end of the belt has no holes.
I'm 5 foot 2 inches tall (or short depending on your outlook
). When I sit on the floor the distance between the floor and my neck is about 21 inches so I could get my head in the loop but it wouldn't be forcing me to remain erect to stop the pressure. Someone taller than me definitely wouldn't be straining at the constraint. I have no idea how tall the Shermans were. I saw an image of them with a group that included Anderson Cooper. He's listed as 5 foot 10 inches and Barry looks about two inches shorter than him and Honey two inches shorter than Barry and she's wearing low heels. That would make her about 5 feet 4 inches tall.
I doubt they were alive when placed in the loops of the belts because they would be struggling mightily. It would probably also require more than one person involved in their death from a practical point. (I'm not saying just one person was involved, though). I'm pretty confident the dead weight of both kept the bodies upright but in no way contributed to asphyxiation.
If each belt was looped individually the same scenario would apply by looping the belt around the railing first then cinching it. And then placing the belts around the Sherman's necks to keep them upright and facing a blank wall.
I definitely think the whole scenario was a message of some kind; a personal one. If it was a professional hit, you know, get in, neutralize subjects and leave type of scenario, the Shermans would have been found anywhere else in the house, dropping where they died. But this just reeks of some personal vendetta and one that satisfied some deep seated hatred toward them for some hurt, real or imagined.