I don't have a lot to add here, just a few summary points.
1. The time to get comfortable with doubles is not in a class. You can learn the basics of diving doubles on your own (mileage varies and not always the best approach), with a knowledgeable mentor or in a class that specifically teaches a doubles configuration. But while having a half dozen dives in an intro to tech class will speed the learning curve, it won't get you comfortable in diving doubles. That takes experience over a variable number of dives.
2. Getting "comfortable" in doubles needs to include getting properly weighted, mastering precision buoyancy in them, mastering a level trim position and getting comfortable with all the normal OW gas sharing skills and emergency drills. Then and only then are you are probably ready to take the configuration into a class room situation, like cavern, intro to cave or AN/DP.
3. If you attempt to do a class like AN/DP in a new configuration you will be using most of your band width trying to manage the doubles and are going to be so task loaded that you are not going to learn much from the course.
4. An instructor who would let you take AN/DP with doubles with basically no doubles experience is an idiot. Don't walk, run away from that moron. All the other failed to properly manage the class issues just add to the reasons to leave him in your wake.
5. AN/DP procedures is, or at least should be, the most intensive course in the TDI or IANDT technical diving course progression, but it is not really priced that way and many instructors may not view it that way. But the fact is that you should be mastering all of the basic technical diving skills (valve drills, deco bottle use, gas sharing, bag shooting, etc) and that in other more advanced clases, such as normoxic trimix, you are just building on those skills, adding a gas or bottle and extending the depth range or amount of deco obligation - the basic skills are the same. Price wise, AN/DP should cost $1500 and normoxic trimix $500 to $750 - not the other way around since ANDP involves more dives, more classroom/equipment configuration time and instruction and mastery of ore skills. The moral being, students shoudl nto be encouraged to take AN/DP until they are truly ready for it, rather than having AN/DP be regarded as a gateway to tech course.
6. You are ultiumately responsible for your own safety. Don't blindly follow anyone anywhere, even an instructor. Choose instrcutors very carefully, interview them and interview former students to try to find out exactly what you are getting and identify the instructors strengths and weaknesses up front. The fact is, there are a lot of instructors out there and soem of them are really bad. In some cases, what they believe is pretty suspect, and in others they may be very solid technical divers, but that does not mean they can actually teach technical diving effectively.