It is absolutely possible. There are guidelines about weight gain, so weigh yourself once a week to see where you are.
For me, it's not about dieting, it's about healthy eating. Think about nutrition...how much nutrition are you getting from the food you're about to eat? If you're eating a chocolate bar, that's taking the place of something else (eg apple) which would be much more nutritious. That mentality helped me stay within guidelines.
There is no need to eat for two. Eat until you are satisfied, then stop. At some points during the pregnancy, you won't be hungry. That's ok. At other points, you will be ravenous. Listen to your body. (if you are LOSING weight or are concerned at all about your appetite then talk to your doctor)
Do you exercise? It helps to keep up some level of activity, starting as early as you can, and gently at first if you're not used to it. I found weight training, elliptical and lots of walking worked well until 3rd trimester. Then I swapped to just lots of squats to help with the birth itself, and arm exercises to help with carrying the baby/car seat etc. There are specialist trainers for pregnant ladies, it's worth going for a session or two if you can, or watch youtube videos of Tracy Anderson etc.
After birth, it takes roughly 6 weeks (varies a lot by person) to be able to move comfortably/bend down etc. You will be very grateful after birth if you have given yourself a head start on fitness etc. To get back into shape, there are mummy and baby fitness classes which are a great workout, tailored for postpartum ladies, and are a good way to meet other mummies IME.
Good luck. It's totally achievable.