Battery trade offs mah vs weight of multiples

I purchased a 3S 5500mah 40c lipo for my Tarot 690 hexacopter with 750kv xinte 5010s , I get 12 minutes give or take. I have just ordered a parallel cable and
Two 3S 8000 mah 45c lipo which I am planning to parallel. With 1 battery 8000 now compared to my 5500 original what can I expect for flight times with the 8000mah. Also does the weight cancel out the second battery? Would twin 5000 be more efficient than a single 10000mah? Any guesses how much time I might get with 2 8000s? Thank you