<< Solved >> Dual monitor setup not working with SMBIOS iMac 19,1

[darrenkhouston]

I too am running into this same issue. I already have iGPU disabled, and I recently updated to Catalina. I can get mine to work on iMac 14,2 but if I try it with iMac Pro 1,1 or anything else, only one screen will work. It was working previously with Mojave just fine on both screens with iMac Pro 1,1, but since the update, I had to go back to iMac 14,2 in order for both displays to work. This is using two DisplayPorts on the card, not using HDMI.

 

[vinodh]

I had it fixed with the GM Seed but after I updated to the public release, it's shagged again. The only change I made was to add OsxAptioFix2Drv-free2000.efi. Everything worked without this but I couldn't get the update installed without it.

So, i'm back to one monitor again...

 

[foster]

I only have one UHD display on my system but it has three inputs: Display port and 2 HDMI
All ports on RX 580 work under Catalina

 

[vinodh]

I left my system alone since Tuesday night. I didn't even switch it since then and today, BAM! both monitors are working again. Makes no sense.

 

[vinodh]

I updated to the new supplemental build and got the error again. I figured out that what fixes it is booting to Windows and then booting back to macOS.

I’m not sure why this works but once I’ve done it once, it remains fixed until the next update.