I haven’t tried it myself, but I would be stunned if they didn’t let you do this, as long as both parents are Platinum. One uses one of the certificates with one child and the other uses the other one. If they give you a hard time, I would ask them to push two tables together.
But maybe I’m wrong. If there are two cabins booked, the best way to do this is to put the Platinums in separate rooms, getting four total meals. Key cards can be changed onboard.