Your brain can handle a total intensity index of 100%. Normally you take a 100% dose to reach that. If you want to combine two drugs then you need to lower your dose so that the intensity is not excessive.
For example, if you usually take 3g psilocybin mushrooms and 150ug LSD then you would go for 50% of each to give 100%: 1.5g shrooms and 75ug LSD.
Sometimes for drugs which do not directly overlap, like LSD and MDMA, you may wish to combine them to exceed 100%.
"You can always take more but you can never take less."