Q: (Asks why E + Y together might be dangerous)
A: To understand my answer I need to give a bit of background physiology (I'd note that this is covered in more detail in both my Ultimate Diet 2.0 and the new Stubborn Fat Solution, especially the latter). Hormones such as adrenaline and noradrenaline (aka epinephrine and norepinephrine) have specific receptors in the body which are called adrenoceptors (or adrenoreceptors). When those hormones bind to their receptor, stuff happens (to put it into technical terms).
There are two broad classes of receptor which are beta-receptors and alpha-receptors. Each has several subtypes including beta-1, 2 and 3 and alpha-1 and 2. Adrenoreceptors are found all over the body in various concentrations depending on the tissue.
In fat cells, beta-1,2 receptors are critical for fat mobilization, in the heart, beta-1 receptors are involved heavily in raising heart rate and blood pressure. In fat cells, alpha-2 receptors inhibit lipolysis. In the heart, alpha-2 receptors slow heart rate and blood pressure.
An analogy I've used before is that you can think of the beta-receptors as accelerators, when you activate them, certain processes increase (such as heart rate or fat mobilization). You can think of alpha receptors as a brake, when they are activated, stuff slows down.
Now, ephedrine works both through weak binding to the beta-receptors (which is how it both increases fat mobilization and increases heart rate and blood pressure); it also works by increasing the body's output of adrenaline/noradrenaline (which do the same thing). You can think of this as pushing down the accelerator (to stretch my analogy quite thin).
The other compound you mentioned, yohimbe works as an alpha-2 receptor antagonist, meaning that it blocks the effects of the alpha-2 receptor. You can think of this as taking your foot off the brake. By inhibiting an inhibitor, yohimbe ends up increasing lipolysis and heart rate/blood pressure.
So what happens when you combine ephedrine/caffeine (press the accelerator) with yohimbe (take your foot off the brake)? Well, while this can have some very nice effects on lipolysis, it can also shoot heart rate and blood pressure sky high. Which is why the combination is potentially dangerous and why I generally recommend against it.
And yes I know folks who have used them together. Usually these are folks who run low blood pressure to begin with or who are at the end of a long diet (when nervous system activity is down already). In that situation, the combination of yohimbe/EC may only bring blood pressure up to normal and won't cause a problem. But in folks carrying a lot of fat (where high blood pressure is typically already present), I wouldn't even consider combining the compounds. It shouldn't be necessary in the first place and could be downright dangerous.
Also going as high as 0.2 mg per kg is only if not used with caffeine; I wouldn't ever go that high with caffeine. 
