Now I've always thought this rumor was completely untrue because not only was there never a source for this information, but to me the plot has always held together rather well. It's true that there are plot points and scenes that were never fully explained in the anime, but this is filled in with supplemental information provided on the official Japanese website (it was also included in the booklets that came with the blu-rays). I can certainly see how people might think the show was originally planned to be longer, but the script always seemed consistent enough to have been planned as 1 cour from the start. And, this turns out to be the case.
Secondly, the official Design Works book contains an interview with the Symphogear script writer and co-creator: Akifumi Kaneko. Thanks to an Anon on 4chan, a translation has been provided that explains how the series was originally created (I've edited this a bit for better grammar):
I've read almost all of Symphogear's staff interviews and they never mentioned anything about the script being halved. This all started because Agematsu wrote too many ideas into his notes, so much that it was impossible to put into a 1 cour anime. When they started, King Records and Bushiroad couldn't and wouldn't give them enough budget to make an 2-cour anime because Agematsu (a composer) and Kaneko (a game designer) didn't know about what recent anime are like at all. You see? THEY KNEW NOTHING ABOUT HOW TO MAKE AN ANIME. 5 years ago there was a producer who asked Agematsu a question: "You make so many great anime songs! But are you able to write an anime story?" After that question, Agematsu started to think about how to write a story for an anime and wrote all of the ideas into his notes. (According to the interview and Agematsu's twitter, there were originally 5 heroines, 4 main enemies, one pet, a book of the prophets and a final boss who wanted to control the moon to make an infinite phenomenon of a solar eclipse.)In 2009, at a party of game developers, Agematsu met his friend Kaneko and asked him how to write a story. Kaneko didn't know how to teach Agematsu to write a story, so instead he took Agematsu's notes back home and spent one week to complete a plot, which almost fulfilled all of Agematsu's desires. In reality all Kaneko wanted was to give Agematsu a sample of how to write a plot, but because the plot that Kaneko wrote had fulfilled Agematsu's wishes, Agematsu started to write the proposal for Symphogear to King Record's producer and tried to persuade Kaneko to join him. But Kaneko didn't want to join the project at first, and said if he's the producer he won't let the proposal pass. But Mishima, the producer of King Records, let it pass just for one reason: He and Kaneko had worked together a few years ago and Mishima felt it must be fate that they get another chance to co-work again this time.
So in the end, the rumor was born from the fact that Agematsu had drafted enough content for more than 1 cour of anime, but never in the proposal process for Symphogear was it ever planned to be longer because they didn't have the budget from the sponsors. Honestly, reading through this story it seems like a miracle that Symphogear even got funded in the first place! But it also explains why Symphogear's plot seems different from other anime - it was written by a person who's experience was mainly in JRPGs.
|This is the part of the interview where Kaneko talks about Agematsu's original story ideas|
Hopefully this will finally put an end to this rumor. You can read the original 4chan posts here and here.
If you have a copy of the Design Works book you should definitely check out the interview! I'd like to provide my own translation someday, but it's a pretty big undertaking and I'm busy working on the second volume of the manga! Look forward to chapter 1 soon!