If you have Leviton plates, you should stick with Leviton, ICC or Hubbell inserts. In my experience, the Leviton plates are very likely to break when using AMP inserts. If you had ICC or Hubbell plates, they stand up to the AMP inserts a bit better.
As for fitting two together, JWRacedog was referring to surface boxes. If you have a two port face plate, they will fit together without a problem. So long as the plate doesn't break in the process.
