They might just be taking too much at once, at only a week old, and being twins they may be smaller than a typical single baby, their tummies are VERY small, like the size of an apricot - stretch it too far by overfilling, and it will push all the food up and out. Burp every 1/2 ounce. Mine could only drink 2 ounces at that age, without spitting up alot. Of course, they drank about every 2 hours, only 2 ounces....and yes, like someone else said, keep them semi-upright for at least 20-30 min after they eat, in someone arms, in a swing or bouncy seat or a carseat carrier, with the handle all the way turned backward under the seat and locked in place to prop it up.
I definitely don't think warming the bottles will help or hurt on the spit-up issue. However, if I were you I would stick to room temp, much easier for travel etc. if you don't need to heat everything, keep them used to that. Mine would not drink room temp or cold breastmilk or formula, since they were used to nursing and that is HOT from the tap, LOL.