A stop is the corner web of your mouth. If you were to get your tongue skinny enough to block out ONE note and play a note on each side, this would be a double stop. WHEN the webs at the corner of your mouth only let the note on each side sound and what you get is TWO notes (one on each side of the one you blocked with your tongue). Since there are TWO notes, this is a DOUBLE stop.
If you were to block one middle note and sound either ONE note on one side and TWO notes on the other side (doesn't matter WHICH side) this is ALSO a double stop. Reason? The 'double is done by the webs at the corners of your mouth.
If you were to block out TWO central notes, and play one (or MORE) notes out EACH side, this is STILL a double stop. In other words, ANY combination of notes qualify as long as they are on both sides of another note or series of notes.
eu mesmo nunca soube direito o que era double stop ;-)