With any non-metalic box, it's use in a fire rated wall depends on it's UL listing. When it comes to installing boxes in oposite sides of the same rated wall, the separation required varies greatly not only between manufacturers, but between different boxes by the same manufacturer. This information can be found by going to
The UL web site and searching for UL categories QBWY and CEYY. I was able to find information for Allied boxes which can be spaced 3" apart as Redsy stated, but the wall must contain bat insulation.
I remember inspecting a motel where the electricain had used a variety of boxes by different manufactures. There were 6 or more different standards that needed to be applied throughout the building on a case by case basis depending on which box was used at a particular location. What a nightmare. My advise is, settle on a particular type of box that you will use throughout a project and stick with it. Find the UL information on the installation requirements for that box and use that information. Have it available for the inspector when he come.
