Mixed Up by Gordon Korman

From the author of Restart, the story of two boys who are losing their memories . . . to
each other.
Reef and Theo don’t know what’s happening to them. They’ll be going about their days and then
suddenly they’ll have these strange flashes of memory — but the memories don’t belong to them. And at the same time, their own memories are starting to . . . vanish.
For Reef, this is a big problem, because memories are all he has left of his mom.
For Theo, it’s strange because the new memories give him a freedom he doesn’t have with his domineering dad.