You don't need anything other than Savarese. I have another book (that the authors gave me to review) and it didn't have anything that Savarese didn't and was missing a lot of what Savarese has.
The autonomics pages in Savarese is golden. Know them well.
I've had Chapman's point questions that weren't covered in Savarese, but basically everything else is there.
I'd spend time going through the content of Savarese first, again, special attention to autonomics. Be sure that you read through the cranial stuff & anything you're not comfortable with the mechanics of. I'd then try to do one or two of the tests at the end of the book. You may or may not see questions like those, but they give you some idea of what types of questions can show up. I definitely wouldn't do all of the tests in prep for step1, though. Savarese is useful for steps 2 & 3 prep, too, so you can save some for later. (although all I really did for step 2 OMM is to review the autonomics and glance at cranial)
All that said, you shouldn't have to spend more than 1 day prepping for the OMM.