>spontaneous
Yeah, usually when one person started speaking in tongues, the other would follow suit almost immediately after. Then it became a game of which one would start it first from one Sunday to the next. That's usually how you could tell who was the most faithful and what not.
It was intentional. Now the question is "Why?".
Yes, one to interpret/prophecy the tongues, and another to test the prophecy to ensure it actually came. I'm not sure why Jesus dint just send everyone the same message while they were in the spirit to ensure there were no misinterpretation, but his ways are mysterious and all that.