Is it ok to feed them normal formula milk?

I am going out for the day tomorrow and I have a flask that will hold hot water for 2 bottles. I have twins, so basically that is one feed. I will need to give them another feed after that while I am out. The thing is they are on special formula milk for prem babies that you can only get on prescription. What I want to know is, is if I can buy two of the ready to feed cartons of normal formula milk that you buy in shops instead of their normal milk they have. They are 4 months old