Does anybody have any idea how to spawn 2 items on an interface like this? Clearly it's possible from this really old youtube video when the client was still around the 317 revision. I have the itemoninterface packet of course, but maybe I'm missing something to display 2 items like this?

You just display 2 instead of 1 mate

you sure thats the right packet your trying to figure out? it may not be item on interface (which is using an item on interface) eg. Alching.
its probably an actual interface being shown with the image on it