I'm suprised you were able to get the backs out without pulling the bottom first...
To pull the bottom, run your finger about one foot inward from the outer side of the seat under the bottom until you feel the plastic clip. There are two of them, one on either side. Use a flathead to poke in the clip and release the clamp. After you pull the bottom seat you should see the two side brackets for the upper portion... but wait... you already pulled a Hoodini and got that out first... :SNSign: