I don't know how others are teaching it. But one of the concepts I find sometimes difficult to help people wrap their heads around is how to read pressure on sidemount.
Bar or psi is irrelevant. What I mean is that you have two pressure gauges. One, obviously, on each tank.
The way I think about it and the way I teach it is to think of those two tanks as a single, unified gas source. Imagine them manifolded together, but without the stupid UTD manifold that neither works properly nor should it even exist in a rational universe. This is just an imaginary manifold.
This is where people frequently get confused:
"If I read 100 psi on this gauge and 100 psi on that gauge, it's 200 psi?"
In that single, unified gas source 100 psi on each gauge, having breathed 100 psi from each tank, means you have used 100 psi.
100 psi from a single tank means that you have only breathed 50 psi from your unified source. But you might as well just avoid such a complication by balancing your tanks as closely as you can. (Personally, I prefer not to let my tanks get further than 10 bar/ ~150 psi from one another.)
Think of the needles on your pressure gauges as sharing quantum entanglement. When the needle on one moves, so should the needle on the other. The two of them, moving together like that, mirror the behaviour of a single needle on a single SPG on a set of doubles.
It would probably help if you understood quantum entanglement.
Come to think of it... it would probably help if I understood quantum entanglement. I don't know how it would help, exactly. I just wish I did.
I don't particularly wish or care about understanding the UTD manifold. But I still do wish I had one for The Shrine.