Wait and take the heroin when you come down or the next day.
Mixing psychedelics/MDMA with heroin is a waste in my opinion. It mostly dulls the experience. and you'll still be tripping but tripping like 'blah, sleepy but not sleepy, tripping but not tripping...meh' kinda trip
If you're addicted to opioids, it is safer to continue to take a maintenance dose while tripping.. you don't want to experience withdrawal on a psychedelic. Otherwise yes, don't combine. These RC psychedelics don't have really established safety unfortunately.