Both GPU cards are the same Model and both support Crossfire so it should work if the Motherboard supports MultiGPU or Crossfire on it.
List of AMD Crossfire Supported GPU Cards: List of All AMD CrossFire Graphics Cards for PC
Here is the list of all desktop graphics cards from AMD that supports CrossFire technology.
Update: AMD has officially ended its brand CrossFire for multi-GPU configurations. Now it will be called multi-GPU or mGPU and just like Nvidia, newer graphics cards from AMD will only support a 2-way multi-GPU setup. This is applicable from AMD’s latest Radeon RX Vega 64 and RX Vega 56 graphics cards.
FYI: Here is a list of all AMD Crossfire or mGPU support games: Crossfire Game Compatibility List | AMD Crossfire Wikia | Fandom