^ This.
It's not advisable to mix benzos with any other depressant. This has been the cause of countless deaths.
If you're going to mix the two and cannot be deterred, you need to take half your usual dosage of buprenorphine and a very small amount of benzos. However, even though mixing benzos and opiates/opioids is dangerous and even fatal many people report that mixing does not result in making you higher. In fact, the benzos actually can dull the opiate euphoria according to people who have taken the two together.
Sorry, but we're not here to help you get high on their prescribed medication. We can only tell you what's safe and what's definitely not safe. Buprenorphine has a ceiling effect at high doses. That means that taking higher doses produces no additional effects. So taking high doses to get high just won't work and you'll just be wasting your medication.
But yes, Tagamet is effective in potentiating your bupe. Just take your normal dose though, just like NT said.