Hah, I couldn't resist. As at least one forum member can testify, I have two names and go by both, yet people insist on calling me by my second name and mispronouncing it to boot.
RYBs on names:
There are two types of names – the name you were given when born – known as the shem arisah - and the name you are known by. The shem arisah has a halachic validity, as it is always your name, even if you stop using it. On this, Rav Moshe ruled that a name is only a halachic shem arisah if the parents always planned on using it. At this point, it is one’s name even if they changed it later. In contrast, if parents intended to only use the name when it was first announced at the krias shem, but then never use it again, Rav Moshe rules it is not a name. So, if non-frum parents gave a person a Hebrew name to make their frum grandparents happy, but never planned to use it, the name has no halachic validity. Despite this, if one ever used the name for a mi sheberach or Aliyah, we can rely on that and assume it is indeed a valid name. There must still be some intention that it be used – lechatchilah, one should call the child by that name at least once in a while.
What happens if the name is changed? It used to be very common that preschool teachers changed names if they didn’t like them. The family would give the name Elka, but the teacher didn’t like it so she changed it to Ilana – and it stuck. What is this girl’s true name? Indeed, this problem was so common in England that R’ Yechezkel Abramsky ruled that a get should have only the name in common use, and not the shem arisah – because it was impossible to determine what the original shem arisah was. (In truth, a shem arisah – when real – is never bottul. However, in this case, he said that it was impossible to determine what the real shem arisah was.)
In general, Rav Yaakov Kemenetsky said that it is not so good to give more than one name, as it can lead to problems when only one is used, and the other is forgotten.
For the rest, wait for the book.
