The Turbo boosters are not approved for aircraft use. The Marathons are approved ONLY if you have the 5-pt harnesses still in place and use them. The Marathons are a bit on the large side, however; so two of them in one row might not fit properly.
The FAA approval sticker that you have heard referenced does NOT actually mention the FAA. You will find it on the side of the seat shell (usually near the bottom), and the wording is mandated by Federal law. It will read: "This restraint is certified for use in motor vehicles and aircraft."
Be aware that if you have two kids in carseats AND two adults in the party, the FA is likely to insist that you sit M-W in two rows. Even though it is technically legal to put two carseats in one row, most airlines set policy against it if there are as many adults as carseats in the party, because it is safer in an emergency situation to have each adult take responsibility for getting one child out of his carseat and off the plane. Policies like these probably will be applied to CAReS harnesses, too, because children are presumed to need help getting out of them.
Just FYI on seat assignments: carseats are not only forbidden in the exit rows, they are also not allowed in the rows directly in front of and behind the exit rows.