To do a 'Unison Raid', do you have to have the same type of magic?
The definition of Unison Raid is


  An extremely powerful ability that allows two Mages to unite two kinds of Magic similar in nature together, thereby creating an even stronger attack.


The Wikia page listed Unison Raid that has been performed in the series. We have:


Lucy (Aquarius) + Juvia = Water + Water
Gray + Juvia = Ice + Water
Wendy + Lucy (Scorpio) = Wind + Sand
String + Rogue = Light + Shadow


All of the elements that they use doesn't go against one another. In video games usually Light and Dark/Shadow are opposite elements, but in nature, whenever there is light, there is darkness/shadow. So in nature they are not opposites and in fact are quite similar.

Now, there is a problem with what you stated in your question. Mirajane doesn't use song magic. Her magic is a Take Over (transformation) magic. When Mira sings, she sings normally, not with magic, so you can't Unison Raid that.

Is it possible to Unison Raid 2 song magics? Yes, it is. But both mages needs to match the frequency of their song so that the resulting sound wave doesn't cancel each other out. So if the first mage use a song magic that results in the following sound wave (a sine wave),



Then the second mage's song magic must create a sound wave that have the same frequency (same position of hills and valleys). They may have a different amplitude (height and depth of the hills and valleys), it doesn't matter.

However, if the second mage failed to make a sound wave of the same frequency, let's say that he created a cosine sound wave, then the resulting Unison Raid wouldn't be as strong, if it can be called a Unison Raid that is. It is even worse if he created a -sine wave. They will cancel each other out.

For comparison:

Not very synced


Cancels one another


Perfectly synced