I've already posted the problem explained above in a MOS Discord group. Skateman and Geennam have kindly given their opinions and tried to help. Now I'm posting it here for your record in case there's a solution, and also because I think there might be more X5000/040 users here.
I think really is not a problem of MOS or AOS4, it's a problem of X5040 bios or similar.
The problem is that I want to use a RadeonRX550 as my primary graphics card for AOS4 and a Radeon X1950Pro for MOS, just as I've seen other users do with X5020.
The first graphics card goes into PCIe 16x, and the second graphics card uses a PCIe 4x or 1x to 16x adapter.
I've already tried three different adapters: two PCIe 4x to 16x adapters and one PCIe 1x to 16x adapter, and none of them work.
I'd tested both cards connecting only RX and use with AOS4 and connecting only Radeon X1950 to 16x pcie with AOS4 and MOS and works too.
Aditionaly I'd try with classic Ati Radeon 9250 PCI as second card for MOS as temporal soluton but not works.
When I use only Radeon X1950 at startup computer graphic card blow go to 100% speed 2-3 seconds until all X5000 hardware is initialized, when X5000 boot logo screen appear blow speed down to 10%.
When I'd try to use X1950 as second graphic card with pcie adapters at startup computer blow go to 100% speed and never stop. Sound like a X5000 boot sequence can't find second card or can't initilize second graphic card.
It's a firmware or bios defect or exist some command/vars in uboot to be enabled for use 2 graphic cards in X5040 ?.
Someone here with X5040 and 2 graphic cards working?.

Edited by kikems on 2025/8/12 8:47:16
Edited by kikems on 2025/8/12 23:00:25