The 2 topics that you get on the exam won't be the same, i.e. you won't get 2 structure-based topics, you'll get 2 out of the 3; so you'll get a character-based and thematic-based topic, or character / structure... you get the point. So don't stress too much, there'll be at least one topic that won't be on structure!
But another thing about the structure question they ask is that it may actually work to your advantage. I read a topic today that was something like 'The Reluctant Fundamentalist is presented as a one-person monologue. Discuss the effects of this.'
So you can pretty much choose whatever effects you like, i.e. Because of the style of the novel from the point of view of a Pakistani it is anti-American, or that it is an eye opener to different stereo types... once again, you get the point. So as long as you can write maybe your first paragraph on what the structure basically is, then you can choose to write about whatever effects you like. Hope this helps.