According to ADP's website, it seems there is no Longchamp boutique at 2E:
http://www.aeroportsdeparis.fr/ADP/e.../longchamp.htm
I'm not sure there is a huge discount anyway. I've found the following prices for a Gérard Darel handbag (source: ADP website).
- List price: 276 €
- Duty Free price: 243 € (-12%)
You might get better deals at their boutiques in Paris during the sales season.