From what I've heard (I'm just starting this process as well as a DM with ~500 dives) - tech diving instructors can take you out for a day and teach you certain aspects of diving. For example, my instructor is perfectly willing to go out to a quarry with me and teach me how to dive doubles, not under any agency, but just how to dive them in general. That way, when I hit tech, I can be proficient in using them. Perhaps this is the route you should go... Generally you have to pay an instructor for this, but this way you can be used to doubles before having to do more intense dives in them...