If the Radeon Graphics GPU is listed within Crimson, then it is Installed and Fully Usable in Scenarios that support it.
It will however list as (Disabled) if there is no Display Attached (DVI, VGA, HDMI, DP) ... although I should note that the order that Windows recognises the Graphics Cards and Monitors might actually cause issues unless you're using Multiple Desktop Environments in Windows 10 (Anniversary Edition or Beyond)
Personally I would recommend only connecting Displays to your intended Primary Graphics Card (in this case your RX 580) to prevent conflicts.
Also while both GPU are reasonably close in performance, they aren't recognised as "Crossfire X Compatible" ... so don't expect Crossfire Mode to be available.
They can however be used in DirectX 12 / Vulkan / Mantle, Multi-Adapter Mode:
This DirectX 12 Runtime will run if Multi-Adapter Mode is Available.
At present it doesn't support APU and Discreet Graphics Combinations, this is due to a Driver Bug... if you want to test with APU R Graphics and Discreet Graphics, then you will have to Clean Install Radeon Crimson 16.9 or earlier.
You should see this when it runs, the Primary GPU Renders the Triangles in the Scene; then each Additional GPU (and it doesn't have to be a Radeon, any DirectX 12 Compatible GPU will be detected and utilised) will handle a Blur Pass. The Above Image is with an RX 480 + R7 370 that I have installed at the moment.
Hopefully this will show that your R9 290X is fully installed and enabled, but when / if it will be usable as a secondary GPU in Games is another question entirely that I'm afraid I can't answer. This said, you should be able to add a Second Monitor to it; and then the Desktop it runs will be entirely Independent of the Primary Display which will help with Gaming Performance when in Fullscreen without loosing the ability to render and use both at the same time.
Though I'd add this as well... because something to note is the PCI Express Bus Settings.
(Check the Crimson Settings > Settings Tab)
If your R9 290X isn't on a minimum of PCI-E 2.0 x4, then it will not work.
Some Motherboards will have a secondary PCI-E x16 Slot; and these will typically be at x4 Lanes but they might Default to PCI-E 1.0 / 1.1 instead of 2.0 due to a lack of PCI-E Lanes.
Ryzen CPU have 16 PCI-E Lanes, with the A320 / B350 providing an additional 8 Lanes (PCI-E 3.0)
Previous AMD CPU (and many Intel i3 / Pentium) will have 8 PCI-E Lanes, with 4-8 Lanes Provided by the Chipset (PCI-E 2.0 / 3.0)
This typically will not be enough for driving Multiple Graphics Cards that require PCI-E 2.0 or Better.
Since you have not provided the information requested, I am locking this thread.
If you are still experiencing this issue, feel free to open a new thread, following the guidelines here : INFORMATION REQUIRED WHEN POSTING A QUESTION