I think you will have a problem combining good performance with 12 minutes of flying.
12 minutes is a long time, and would therefore require large batteries, which again would increase the weight, requiring even more power etc.
Most electric pattern planes fly only about 8-9 minutes, and I think this is a better target.
Considering that it takes at least one hour to fully charge most Li-po packs, having only two packs will seriously limit your flying. This wil also depend a lot on how you charge the batteries, what kind of charger(s) and power source you are using etc.
I have had anything from 1 to 5 battery packs. I feel limited with anything less than 4. This is my opinion only, others may feel differently. (Particularly those who don't fly electric, but have read about it.)
For F3A performance, it is normal to use 10s packs, with a capacity of about 5000 mAh. Models weighing close to 5 kg. With a model weight of 3600g, using 8s, you would probably be looking at 3700-4000 mAh for reasonable (sport) performance. 1100 sq. in. is a large wing area for a 3800g pattern model. This area is actually larger than most full size (2m) pattern models weighing 4900-5000g.

Magne