Benga asked SWIM to clarify cocaethylene toxicity vs normal cocaine. Both are toxic in the same way by (1) restricting bloodflow to heart muscle (and everything else), by (2) increasing the rate and strength at which the heart beats, drawing more from bloodflow which in turn is being decreased (ie - oxygen, sugar, etc), and (3) by its effects on heart rhythm. Cocaine, lidocaine and all the other -caines block sodium and modulate other ion channels, slowing the conductance of impulses across the heart's pacemaker. If this blockade is too intense, it triggers abnormal heart rhythms or stops it entirely.
Cocaethylene is more toxic because it is more potent than cocaine specifically at boosting dopamine and blocking sodium channels. Cocaine also modulates serotonin, and the serotonin decreases the effect of the dopamine, but this effect, in contrast, is absent with cocaethylene. So one can get higher with cocaethylene, but one's heart is also more likely to fail and at a lower dose.
There is no novel mechanism of toxicity with cocaethylene that is not present with cocaine, however.
Lastly, ingesting alcohol with cocaine allows the toxic effects of both to be exerted at the same time, so the presence of ethanol has its own additive toxicity.