I have, but not with a Mountain House. There are pros and cons to doing it. The pro of course is you gain some space, the con is that you eliminate the bag in which you rehydrate, so you have to have something with you in which to rehydrate, and whatever that something is has to be cleaned.
Most recently, I have used dehydrated soups by Alessi. I repackage those because I can get two meals out of one pouch/bag/whatever (and the pouch isn't a cooking pouch anyway). I double bag the portions in quart freezer zip-locks. I heat the water in a metal bowl, then rehydrate in a Sea To Summit Delta Bowl (with lid). It has to sit a good 15 minutes or more to really hydrate everything, and I eat right out of the Sea to Summit bowl. Getting the lid off is a bit of a pill after that time, slow and steady or dinner is on the ground.
The difficulty in dividing the pouches into portions is that the seasonings/salt settle differently than the main ingredients. When I divide the pouch into two portions at home, I use a sifter to segregate the seasonings from the main ingredients. That allows me to create two portions that are pretty close to equal in terms of main ingredients and seasonings.