how do you make an atttachment another avi can sit on?

i've seen saddles and things for horse/centaur avi's that allow another avi to sit on them and ride around on said horse/centaur avi.

How do you create something like this. adding a pose/sit script and a pose to a normal prim thats attached doesn't work.

They are a vehicle, changed around to suit the task.

Yep - avatars can't sit on other avatars.

ok how do you create a vehicle that someone wears and another person sits in? I find that concept confusing at the least :/

Actually, both avatars "sit" on the vehicle. Movement and the "driving" avatar's animation are controlled by scripts in the vehicle object.

The saddles that I have seen are a prim object with two pose balls. You rez the object and sit on the "horse" pose ball, which operates an animation override to control your pose and movement. Then the rider sits on the "rider" pose ball, which places them in an "in the saddle" pose. The horse essentially "drives" the vehicle, no differently than someone driving a two-seat motorcycle.

Saddles have to be custom made for a specific avatar figure, so the animation places the horse in the right position, and so it doesn't conflict with the animation override for the horse avatar itself that shapes your body and controls your movement. For example, you can not use a saddle for a "Hoof-It!" quad horse avatar on an Extrovirtual pony avatar, or on a Jakkal's Werehouse Gryphon avatar.

To learn to make them, start by trying to make a more normal vehicle, like a car or a motorcycle. There is information in the LSL WIKI on how to build and script vehicles.

Note that a one-person "vehicle" can be designed as a prim attachment - for example a prim riding horse. You wear it like any other attachment. But a dual rider saddle to go with that prim riding horse is still a seperate "vehicle", as described above.

What you may want to do is purchase a saddle and an appropriate avatar, to see how they function. You probably won't get full-perms access to the scripts, but looking at an actual one and using it in-world will teach you a lot.