Verbatim to the title: How can one prepare for BOTH the SAT and the ACT?
I am currently a sophomore planning on taking the old SAT based on the 2400 point system in the fall and the ACT. I plan to start preparing for the tests over the summer, but have had trouble starting preparations for them during this school year. Thus, I would appreciate any advice and productivity tips given. Thanks!
These books will definitely help. There are also forums you can look at here to help. good luck 
I would just study for the SAT and then when you are finished just shift gears and then study for the ACT. I would not try to study for both at the same time. In my opinion preparing for the SAT, which in its current format is a trickier and more thought-requiring test, will put you in a very good position to deal with those issues on the ACT. Then you will just have to deal with changing formats and tackling a few issues that are particular to the ACT (like the science section and some of the Math that doesn’t appear on the ACT).