If you happen to have $1499 USD to spare, the Radeon Pro Duo is now their top-of-the-line graphics card. The Radeon Pro Duo is capable of an incredible 16 TFLOPS of compute performance. With this expensive graphics card, it's water-cooled similar to the Radeon R9 Fury X.
The Radeon Pro Duo will begin shipping in early Q2'16 at $1499. While most Linux gamers/enthusiasts don't have that much to spend anyways on a complete system let alone a graphics card, the Radeon Pro Duo likely won't be worthwhile under Linux. AMD/RTG hasn't offered any review samples to no surprise, but the dual GPU support would make the support currently a mess under Linux. There is CrossFire support with the now deprecated fglrx Catalyst driver while under the open-source driver stack -- including AMDGPU -- there isn't any CrossFire support at this time. The open-source OpenCL/HSA/GPGPU stack leaves a lot to be desired that even with a single GPU it's far from being worthwhile at the moment and so a $1500 investment on the red side for a dual GPU Linux solution would be bonkers. Hopefully the multi-device AMDGPU support will improve in the months ahead.