Always check the cables.
I used to know that. I used to support land surveying equipment for a corporation, and I suppose our cables weren’t all that good, but it was a good standard rule to follow. Always check the cables first, and when traveling to visit customers, keep a few extra cables in your bag to be the visiting miracle worker who quickly solves their problem.
Got reminded of that today, so thought I’d remind others. 
I’d been installing a new video card into my Manjaro computer. I did not expect problems, I was replacing a radeon card that I thought had been having problems and was getting worse, with another radeon card. I half expected to plug it all in and have it work. But it didn’t. I was only getting one screen of my dual screen computer. I tried lots of stuff, different slots on the mb, checking the Bios settings, etc. I thought I had checked the cables, as I found an extra and the problem still remained.
I was down to my last two options. One was post out here pleading for help. The other was to call the card defective and set up a return. But, before I posted here, I decided to go through and systematically check the screens and the cables.
Yep, turns out that of 3 hdmi cables, I had 2 bad ones. I suppose the one I found in my closet to ‘test’ with was also bad. A trip to the local walmart to buy an overpriced but still not expensive hdmi cable, and now I have my dual screens working with my new card.
So, no problem to solve here, but just a reminder of something I should have done first … always check the cables. Posted in case it helps someone sometime someplace …