TANDEM was...okay. I still have yet to read Jarzab's other novels, but I have heard great things about her The Opposite of Hallelujah. However, TANDEM just did not read like a seasoned author's book, the writing was a bit choppy and hard to follow. 
 Ah, parallel universes. They are starting to pop up more and more in YA nowadays, and I have to say that the best book that has tackled this the best was PIVOT POINT by Kasie West. While TANDEM had a fantastic premise, it was weighed down by poor execution. 
 With changing POVs, settings, and even universes that were not clearly divided or announced, this book made me wrinkle my forehead more than a few times. Who was who and what was what? 
 The writing in TANDEM made things even more difficult to understand because a good portion of it was all tell, no show. (She did this, he did this, they moved here, etc) It got pretty bland after awhile. 
 Still, TANDEM kept me reading. It does tell an interesting story, albeit one that is hard to wrap your head around, but I can definitely see this series going somewhere. If you are a sci-fi fan I would suggest giving this one a try.