I had the EXACT same issue and found your question. Couldn't find the answer anywhere so I just held the speaker firmly, with one hand gripping each side of the housing, and pulled. It came apart very easily (be careful not to yank it and damage the wiring!) but having said that a) my speakers are 7 years old now so perhaps the glue degraded a little and b) Over the years the have been battered a little and that one may have been dropped. Even so, I bet if you put a narrow but reasonably robust metal object in there (not sure if any coin would be thin enough)and twist it, I bet the halves would separate).
After pulling them apart, the two halves do still fit together securely enough to stay in place, but only JUST, it takes no effort to separate them. Therefore some glue is probably a good idea, but I haven't used any yet.