When I was trying to sign up for a tec 40 class, I kept getting the run around on actually starting the class. It was listed on the diveshops Website. It turns out the instructor was not actually certified to teach tec 40. Since then, I have always checked with padi to see if the instructor was actually certified to teach the course that I was interested in taking. At first they wanted to know why I was asking that and I had to tell them the story. You could also ask to see a copy of the instructors certification card if you get any funny feelings.
The padi tec 40 course assumes that you are using backmount according to the valve drills and manifold skills. You are not expected to know how to dive doubles competently prior to the course. That is what part of the course is for. Some instructors actually discourage learning on your own prior to a class, as you could easily learn the skills incorrectly or worse.
The course Standards were later modified to allow the course to be done in sidemount, if the instructor was also at least a sidemount instructor. An example of that is reduced times required to do some skills with sidemount.
Tec sidemount was created to develop sidemount skills independent of the individuals tec certification level. For example, in the tec sidemount course, there is no explicit decompression instruction, other than carrying at least one deco tank, and doing arbitrary decompression stops. It is possible to receive both a tec and the tec sidemount certification by simultaneously adding in skills from the tec sidemount course, if the instructor is also a tec sidemount instructor. However, to learn both sidemount and decompression procedures is quite a load at the same time, IMHO.
There are actually some interesting skills included in the tec sidemount course. In particular the regulator removal and switch between tanks exercise on one breath. There is a reason why this is done on dry land rather than underwater. And is is not just because you could flood your gauge.
Some instructors require a student who wants to do a tec course in sidemount to already be sidemount certified or to take tec sidemount prior to taking the course.