I think you mean CrossFire, not SLI.
Navigate to Windows Device Manager > Display Adapters and check that both are showing as enabled.
Please provide more information.
Thanks for the advice but both cards are enabled in device manager. Below is a description of my system. Hope this helps
Radeon Software Version - 16.3.2
Radeon Software Edition - Crimson
Graphics Chipset - AMD Radeon HD 7800 Series ( 2 cards)
Memory Size - 2048 MB
Memory Type - GDDR5
Core Clock - 860 MHz
Windows Version - Windows 10
System Memory - 32 GB
CPU Type - AMD FX(tm)-6300 Six-Core Processor
Rosewill VALENS-700M Modular Design 80 PLUS Gold Certified ATX12V Active-PFC Power Supply
Please try the steps below, which i believe should resolve this issue.
Power off your system and disconnect the second 7850.
1. Download DDU and allow it to remove your currently installed AMD drivers.
2. Download and Install Crimson 16.7.3 drivers and restart your PC.
3. Boot into Windows and shut down your PC.
4. Install the second 7850 back into your system and connect the CrossFire bridge to both GPUs.
5. Power on your system and wait for Radeon Settings to detect the second GPU and install the driver.
6. Restart the system, open Radeon Settings and check Gaming > Global Settings to see if the CrossFire toggle is enabled.