Probably the biggest challenge is that very few treatment facilities will actually work with both of the concurrent issues. If someone, for instance, turns up at a substance abuse treatment program with an eating disorder, the person might be told that they will deal with the substance issue, but for the eating disorder the person will have to go somewhere else. Or if the eating disorder is particularly loud or negatively impacting the person's capacity to work on the substance issue, the person might be told that they're not going to actually treat the person at all, that the person will have to go deal with that first.
It's like a roundabout. It's a revolving door where individuals who are ready for help actually find it exceptionally difficult to get in a door where they are going to be accepted as they are, as whole human beings with multiple difficulties.