Stimulants and Opioids together are no more toxic than either drug by themselves...
However, the danger with stimulants and opioids being mixed is the classic Stimulant+Depressant symptom masking...
Basically, if you overdose on opioids, and take a stimulant with it... You will appear fine, but if the stimulant wears off while you are experiencing an opioid overdose, the respiratory depression is unmasked, and suddenly you just died thinking that your dosage was okay
As long as you don't take too much of one, or the other, you should be fine, there's no waiting time period needed