Honestly? Under medical supervision and at therapeutic levels, generally a safe combination. For fun? I wouldn't mix the two. First, you need to calculate dosage after considering body weight, age, metabolism, and other factors. Are you opioid-naive or do you have a tolerance? Do you have kidney problems? liver problems? All of these things should be taken into account before mixing these two substances. As you probably know, both of these substances cause respiratory depression. Both are considered 'long-acting,' in that they stay in your system for a lengthy period of time. They also have a much slower onset than short-acting drugs, which can lead to someone thinking 'I don't feel anything, I must not have taken enough...I'll take a little bit more...' Many OD's happen this way.
It's ultimately up to the individual, but I'd recommend doing your research.