" Wouldn't it better to attach them at 2 points, especially if a back band is attached to these as well?"

I think so. This is how all commercial boats do it. The idea of hip bracing came from commercially made boats, mostly early British designs that had a suspended seat held by two struts attached to the deck. Suspended, so there would not be the entire weight of the seat (with paddler) pressing on the floor of the boat. These struts also indirectly functioned as braces that kept you from sliding sideways. Personally, I'm not sure that it was really needed. It might have been a way to manufacturer a one-piece fiberglass seat with struts all in one piece for a convenient installation.

If you make some, don't pad yourself in tight. A hands-width between your hips and the padding or brace is about right. You want to be able to turn and rotate freely and you need hip room to do that. It just functions as a brace to prevent you from sliding right off the seat on an extreme lean or rolling, or sculling etc.

You can even just pad the sides of a kayak next to the seat to do the same thing. On my boats I run a brace from deck to bottom and the space between the brace and hull is where I carry my pump.