If we are not talking only Mojave, I too have observed something today about the two cards behaving together.
One question H9826790 at the very first: Is Apple Photos the only App giving the mentioned glitches?
I just updated the firmwares and OS to the newest ones for 3 of my Mac Pros, 2pcs 4,1->5,1 and one original 5,1. I consider High Sierra the newest, not counting any beta software this time.
I've got two Sapphire 8GB Pulses, and one Asus RX 570 4GB. I've got two GT 120 512 MB. I am getting these glithes with Photos all the time.
Mac Pro 4,1->5,1, firmware 0089, MacOS Hoigh Sierra Photos glitches.
- there are glitches with both cards plugged in to monitors, GT 120 and RX 580 Pulse.
- there are glitches with one monitor plugged to either of the cards, both installed.
- there are glitches with only RX 580 installed and plugged to a monitor.
- there are now glitches with my Asus RX 570+GT 120 too. Almost none of this happened earlier.
Glitches behave differently though at times, and a little different with each configuration. If I export the "glitched" pic out from Apple Photos, it seems just allright in other programs- no glitches as of yet:
- I tried Archicad with both RX 570 and GT 120. No glitches as of yet. The picture is fine there, when exported to there, but not in Photos App, it looks shattered, totally red, or like random tv-screen static.
- I tried Affinity Photo with that earlier exported photo - no glitches there either.
- No glitches surfing the shallow web, or using Mail or Preview or any other software, not as of yet. But we'll see.
No glitches observed before these updates, none like that when I was on Sierra and with the earlier firmware, what ever the firmware number was again..
I ain't no software architect:
Drivers acting up against each other - maybe.
Firmware acting up against drivers - maybe.
Apples Metal acting up against whatever - maybe.