"Dan hasn't told us how big he and his partner are, or how hard they intend to hike, so there's no way for us to know how many calories they need."
I proceed from my own experience, as follows: I weigh 137# and have found from experimentation that I burn ~4200-4400 calories/day derived from a combination of ~2700-2800 calories/day/1# 4 oz of food with the remaining ~1400-1600 calories derived from stored body fat which I pack on before my longer trips. I just ran some numbers on www.caloriesperhour.com for backpacking with body weights ranging from 100# to 160# and hiking time of 12 hours. the calorie counts were as follows: 100# body weight = 3810 calories; 120# = 4572 calories; 140# = 5334 calories; 160# = 6096 calories. This roughly correlates with my own experience since I usually hike around 8 hours/day and according to their algorithm I should burn ~3600 calories for that time(.66 x 5334 calories). They do not specify their assumption for pack weight, so there is potential for some inaccuracy there, BTW, but it is in the ball park, IME. I think we can safely assume that Dan doesn't weigh less than 100# and therefore would burn more than 3810 calories, which is far in excess of what he can realistically pack into 18 oz of food, especially given that he is going to be undertaking a fairly ambitious trip with substantial off trail hiking, IIRC. Reducing the amount of hiking time to 8 hours will still put him well above the amount of calories in 18 oz of palatable food. Assuming he weighs 140#, he would have to hike only 6 hours/day to get by on 2700 calories, which can be achieved in 18 oz at a calorie density of 150/oz. Much higher than that and food will become unpalatable and probably unmetabolizable without catabolizing muscle protein over the course of 17 days. This gives me some confidence in questioning whether 18 oz of food would be adequate for a 17 day trip unless he were carrying substantial body fat and the dietary carbs/protein necessary to metabolize it without catabolizing muscle protein.