Meth and Heroin both slow down your heart rate, this can usually cause difficulty breathing which can lead to your death due to your super low heart rate, including how Meth makes you have rapid breathing and you're already low on oxygen, your chance of living is very low from doing one or the other, but both at the same time is an awful idea. Meth also causes dizziness so the chances of you falling over and hitting yourself on something probably really badly could be very high. I've heard these things can make you feel really numb so you might not even notice if you did hit yourself on something but I am unsure on that one, overall it's just not a good idea.