Congratulations on two healthy babies! Sounds like they are doing well so relax.
Each baby is different, my son was up to 8 oz a bottle and back to every two hours by two weeks of age and couldn't seem to get enough so his doctor had us start cereal at that age (which otherwise would have seemed way TOO EARLY). He wouldn't take a bottle with some cereal so we had to spoon feed (and he liked it really thick). Although he weighed only 7lb 15 1/2 oz at birth, he gained about a pound a week for a while and weighed 19 lbs by 3 months. He was such a busy body that by six months, he weighed just 20 pounds even though he ate A LOT. Our doctor was thrilled with his growth and said he was thriving. We were never concerned with him being too big. He is now a healthy 16 YEAR old who is a little on the thin side (not overweight) so I would not worry about your sons being 15-16 lbs at just over 5 months. There is average, then their is above and below average but they are all NORMAL!
My daughter stayed at 6 ounce bottles for what seemed like an eternity and started cereal around three months and liked it thin. She is now a wonderfully healthy 19 month old. She doesn't look chubby like my son did but at a year old was within a pound or so of his 1 year weight.
The 35 ounces a day is not too much for them. At this age, they really won't take more than they want/need. Formula/Milk and Cereal at this age is great for their development.
Mine never only had a tsp or even a tbsp of cereal, it was always at least 1/4 C of cereal mixed with the amount of formula or milk that was needed for the consistency that they prefered. Obviously, you want to know what to do with your children not necessarily what I did with mine.
My suggestion would be to increase their other bottles (those not given with any solids)to 7-8 oz and then you can leave those given with solids at 6 oz. I would also increase the cereal to 1/4 C and give with both the breakfast and dinner feedings. If you children take this amount, you'll know it is not too much. If they don't, you can adjust accordingly.
Best of luck to you and your family.