I couldn't even tell you how I picked the name Sarah. I picked it maybe 25 years ago. Even back then I knew I was a girl inside, and at some point I picked out a name for myself. I remember I tossed around a dozen or so possibilites before I settled on 'Sarah', but I just don't recall how I came up with it. Whenever I've often tried to remember, the TV show "Little House on the Prairie" always comes to mind. I've checked though, and there were no actors or regular characters named Sarah. I really have no idea. I think it was just a popular name that I thought was pretty.
My middle name originates in computers and computer games. Back when I was a kid there was a role-playing game called Ultima IV. In the game you play a character travelling in a fantasy land. Along the way you meet several characters that will join you and help you in your quest. One of them was named Jaana, which I always read as "Jan, ah". I didn't think much about it at the time, but I played the game so much that I didn't forget the name even years later.
In university I took a first year Astronomy class and was paired up with a woman named Jana-Dee. She was very pretty and smart in her own right, but my favorite part about her was her first name. Well... I liked the first half of her first name anyway, I didn't care much for the -Dee part.
A couple more years go by until I bought Ultima Online - a massively multiplayer online game. When I had to pick a name for my character I remembered the name Jaana from the previous game, and I remembered the name Jana-Dee from university, and before I knew it, my character was Jana the thief. Since then I have used the name Jana in most games that I play. After a while I began to consider it an alternate first name.
When it was time to change my name I had been calling myself Sarah in my head for 20-some years, and I had been calling myself Jana in computer games for maybe 8 years. I decided that both names were me, and there was no reason I needed to choose between them, so I picked them both. The world sees them as first and middle names, but really they are just two first names that I couldn't decide between.
P.S. Shortly before I changed my name I began playing World of Warcraft where I made two characters: Saraa the mage, and Janaa the priest. Sometimes when people asked how I picked my names I've just taken the short route and said I named myself after my warcraft characters.
P.P.S. I didn't even consider this at all when I was choosing my names, but afterwards I thought it was cool -my mom's name is Sharon Jean. I think Sarah Jana is a good name for the daughter of a Sharon Jean.