Hello LA Charger;

No I have not seen Rimowa in FRA duty-free, but there are many shops in the FRA terminals that carry the Rimowa products, including the LH Worldshop.

With 2 hours for a stopover, you should have more than enough time to shop and purchase. If you have a specific model you want, it might be a good idea to contact any of the stores there ahead of your arrival. I have done this many time with LH Worldshop (the store in central Frankfurt): With the LH store now in FRA, it saves time.