You can't be on both. If you have a sufficient amount of suboxone in your system, all other opiates will have no effect. This is how I understand it. Opiates attach on to receptors in your brain. Once attached, they allow neurotransmitters, such as serotonin, to flow more than before. Most opiates are full agonist opiates. Basiclly, they open a flood gate and your neurotransmitters flow heavily. Well, suboxone is a partial agonist. It only opens the gate partially.
When you get addicted to opiates, neurotransmittters dont flow like normal, unless you take opiates. You become dependent on them. Without them, you get depressed and everything else that comes with withdrawel. Well, suboxone has a very long half-life and only opens the floodgate enough to feel normal. Your receptors in your brain also prefer suboxone over other opiates.
So to answer your question...when your receptors have suboxone attached to them, all other opiates bounce off and dont get to attach. They get wasted. If you have suboxone in your system and then take other opiates, you will only feel a buzz if there was not enough suboxone in your system to cover all your receptors. When you are taking suboxone as directed, all your receptors should be full with suboxone and flowing normal.
Suboxone will only give you a buzz, when your transmitters are at very low levels. Then, even the little opening of your floodgate, will feel good. But once you are on suboxone, you cant eat more to get a better buzz. It will waste it. This is called a ceiling effect. This is why suboxone is very safe from overdosing. Methadone is not. You can always eat more methadone or other opiates to increase your buzz, but more suboxone will not open the 'gates' anymore.
I know I made this very confusing, but to sum it up....Taking heroin with suboxone is a waste of Heroin! Your brain will grab on to the suboxone and just ignore the heroin.