GUI has a doubles primer course, and TDI Intro To Tech (sometimes called ITT) is basically doubles 101.
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I don't know about switching from a single tank to doubles mid-course (seems like it wouldn't be a problem but definitely a question for the instructor) but after taking Fundies in a single tank, recreational configuration I've been slowly adding the pieces necessary for a tec pass. Whenever I've got the right equipment and feel confident about the stricter standards, it's just a checkout dive with an instructor to upgrade my rec pass and be able to take Tec and Cave courses.
I took Fundies a few months after getting my OW cert and it was hard enough without the more complicated equipment and higher standards of a tec pass. I'm happy doing it in smaller chunks and spending more time practicing, especially since I don't have to retake Fundies to get the tec pass.
Have a GUE Instructor teach you the valve drill for doubles before you start to play with them. You don’t have to take a formal course, an instructors backyard pool and a few hours will help a lot if you don’t want to spend two days
But bad habits are harder to break than good habits are to learn. And turning off all your air underwater is kind of an uncomfortable thing to do and not unknown learning valve drills.