i would think if you really want to run both motors your going to have to go with the bigger bags i think. you can always bump the gas up just alittle. but if you get bags thats to small you will have to run one engine. but also you might want to think about just using one engine. your going to be using alot of gas running both engines. you could pay for a kicker motor in short order running both engines.

with that size boat going that slow your going to have some steering problems with a kicker motor. you might just be as good running one engine with the 48" bags on both sides untill you go to the kicker motor. good luck with whatever you decide. please let us know which way you go and how it works out for you.
sherman