I believe that you can continue to breastfeed the older baby, even after the new one comes along--but if you do, be aware that your older baby may get the "runs" if he has colostrum (first milk), which is designed to help newborns get rid of the meconium (their first poop).
Ask your OB/Gyn about the nutrient requirements. Contact your local La Leche League and ask -- they probably have guidelines & such all written up and they can help with tandem nursing if you go that route.
I'm guessing that right now, you just need to account for the 7m old's nutrition with the extra calories (500 per day, I think is what is called for for nursing moms); as your pregnancy progresses and the in utero baby starts need more nutrition, you'll probably have to up the caloric intake some more, but by then, it's likely that your 7m old will be starting to taper off some on his nursing, too. ... Long story short, start with the guidelines you can find, but evaluate and reevaluate the caloric intake as you go. But eating healthy and being active & drinking lots & lots of fluids should really take care of it--if you're hungry, eat a healthy snack, and maybe go with 4 to 6 smaller meals throughout the day rather than 3 big meals.