Mixing substances means taking more drugs together or even means just add alcohol to one substance. Toxic effects add up, the sum gives increased toxicity. You take substances of which you do not know purity and exact quantity. Mixtures commonly occur among one substance and alcohol, cocaine, prescription drugs, without caring with their sum of toxic actions.
After mixing substances it can occur:
Increased heartbeat

Increased blood pressure

Increased body temperature

Mental disorders and panic attacks, anxiety, paranoia

Alcohol and amphetamine-like substances: you may not notice their toxic effects because they apparently counteract each other.

Amphetamine-like substances and ecstasy: severe dehydration, dangerous high body temperature, heart attack until death.

Alcohol and heroin: slowing of heart rate and breath pace, until death.