Hi Kathryn, I too have been an ileostomate for nine months. First of all, don't listen to anyone who tells you that you can't eat this or that. Everyone is different. I can eat anything, nuts, seeds, skins, all fruits and veg. Eat slowly, chew well, and drink lots of fluids to wash things down. To prevent ballooning, use a pouch with a vent. If you don't want lots of volume in your output, but want to gain weight, eat less high fibre, and more high calorie foods, ie, food rich in oil. High sugar foods will definitely help you put on weight, it's just not good for you! Maybe add a little more sugar to your diet just until you get where you want to be?
I am back at the gym and doing all of my pre-ostomy workout, including abdominals, but I started slowly.
By the way, I also have two ostomies! Mine isn't a double-barrel, but I have a stoma from my original colostomy, which is now not active, and then my ileo which they gave me nine months ago. Because it's been working really well, I'm having surgery at the end of the month to get rid of my colostomy stoma. Good riddance I say. I'll be happy with my ileo and it's improved my quality of life immensely.