It's not "completely okay", there is always a risk when mixing opiates and benzos as the combined CNS depression can be dangerous.
However, that being said, doctors prescribe the two alongside each other in measured doses commonly. I've been scripted benzos and morphine at once before.
I'm curious are both meds prescribed to you? If so you should be fine taking your prescribed doses of each unless the doctor has told you otherwise.
On the face of it, 15mg morphine and 1.5mg alprazolam shouldn't be dangerous. 15mg is a low dose of morphine (assuming it's oral?) and if you have enough tolerance to alprazolam that 1.5mg is a normal dose, it shouldn't cause any issues.
Just be careful about mixing the two in higher doses, I advise against that, stick the minimum doses you need for a therapeutic benefit.