What's the effect of taking blood pressure on both arms at once?

In hospital today, while I was taking a patient's blood pressure, another student asked if she could take her pressure on the other arm to practice. The patient didn't mind, but I thought it might be dangerous.

Is this dangerous? What would happen if we pumped the cuff on both arms at the same time?

Also, would this give an incorrect reading?

Authored By Mohammed Pervez on Wednesday 3rd October 2012