MBP M4 Pro chip cannot drive two external monitors at the same time.

There's one possibility. It's remote, but I can't rule it out.

You say that both of the monitors are ASUS PG32UCDM monitors. All monitors are supposed to have unique IDs, but monitor manufacturers sometimes cheat and program the same ID into a whole batch of monitors. That way, they can use a single firmware image for all of those monitors, instead of having to load a slightly different image (same code, different serial number) for each one.

When this happens, it usually involves monitors of the same type, that just happened to be in the same production batch. So when you buy two monitors of that type,

• You might get ones from different batches, with different IDs, which appear to be unique IDs to your system – in which case you don't see a problem.
• You might get ones with identical IDs, and for whatever reason, a computer might work with them in spite of the standards violation.
• You might get ones with identical IDs, and your computer might not be able to work with both monitors properly, at the same time, because it is relying on that ID, and they are both claiming to be the same monitor.

If you are running into the third case, that might explain the situation. If you could check the ID of each of the two displays (when it is attached by itself), then compare them, that might tell us whether that is the issue.

But I don't know how to view the IDs, off-hand. Maybe someone else would know.

ahearn wrote:

They both display on my M1 at 1440P 100hz same time. They don’t even work at 1080P/60hz together, lid is always closed.

…

i don’t think this could be a monitor issue as they are both the same and both work correctly on an M1 with the pro chip.

I take it that you have a 14" or 16" MacBook Pro with a M1 Pro chip. For one of those, the limit on the number of external displays would be the same whether the lid was open or closed.

ASUS PG32UCDM appears to be a 4K display WITH 10 bits/color options (HDR)

I/O PORTS

DisplayPort 1.4 DSC

x 1

HDMI (v2.1)

x 2

USB-C

x 1 (DP Alt Mode)

That is very near the switching speeds AND total bandwidth for the digital logic used in these cables.There is simply not enough bandwidth to run TWO display on one cable out of the Mac. in fact, there is barely enough to run ONE of your high end displays.

[The possible exception is if you have a genuine ThunderBolt-5 DOCK, which is bleeding edge technology. As a VERY early adopter, you may discover any number of wonky special cases. But you don't get anything faster than ThunderBolt-3 speeds without a genuine ThunderBolt-5 Dock and cables.]

to run ONE:

• you could use a USB-C cable directly and get refresh rates of 60 HZ, you MIGHT be able to get 98 HZ, but not higher. If you turn off HDR, dropping back to 8-bits/color, you could get as high as 120 Hz refresh rate. That USB-C or equivalent cable would have to be ONE meter or shorter.

• The above also applies to using an adapter/cable to DisplayPort.

• 10 bits/color over ONE HDMI cable is much more limited, topping out at 50 Hz, unless you compromise color integrity. if you drop back to 8 bits/color, you can attain 60 Hz, or slightly higher if you compromise color integrity. The required HDMI cable MUST be certified ULTRA cable, which is backward compatible.

• 10 Bits/color using TWO cables: Each cable carries HALF (Left/Right) the display data, and speeds on the cables are far lower, so refresh rate can probably go much higher. (I don't know of any refresh rate lookup tables, you will have to try it) The display puts the two halves back together using its Picture-by-Picture feature. The Mac has supported this for many decades. Users report this feature works great, with no artifacts or compromises.

<< having two monitors working together would be good. >>

Two monitors on two cables works just fine.

Combining them onto ONE cable out of the Mac, is the only part that does not seem to be working properly for you.

a 2K display (2560 by 1440) at 10 bits/color consumes ALL the bandwidth of a USB-C cable at refresh rates above about 200 Hz. there is no additional bandwidth to support an additional display.

a 2k display at 8 bits/color tops out at 240 Hz. but again, that consumes ALL the bandwidth of the USB cable.

He's saying that things don't work with two monitors even if you take the dock out of the picture, and attach both directly to the MBP.

If those are the only monitors, that makes me suspect a "duplicate unique ID" issue, though I can't be certain. It could be there is some difference between the M1 Pro MBP and the M4 Pro MBP – or the macOS versions which they were running – that would account for duplicate IDs causing problems in one case, but not the other.

But that's only a hypothesis. It could be wrong.

ahearn wrote:

Two monitors on 2 cables does not work for me that’s my problem. Only one monitor will display at a time the other shows no signal. Using 2 cables one to each monitor and other end to two TB5 ports on the MBP

If you unplug the dock completely, so there's no chance of it trying to consume any of the video outputs from the MBP – and just plug in the two monitors, does that change things?

I'm guessing not, but it would be useful to know the result of the experiment.

<<. He's saying that things don't work with two monitors even if you take the dock out of the picture. >>

I read,

"That setup is unacceptable to me"

rather than:

"that setup does not physically work"

If that setup does not physically work, how long are the cables you are using?

you don't appear to have the Docks that will solve your problem.

to be successful, a ThunderBolt Dock of any description must already Have the ports you need, like HDMI or DisplayPort, to connect your displays. Splitting a ThunderBolt out of your Mac into additional Thunderbolt ports does not help, because the split-off ThunderBolt ports do NOT have the ability to carry display data from the computer. Only the original ThunderBolt port on the computer knows how to add display data onto the cable.

also, could you please answer again, so that I can be sure I understand:

¿DO two displays on two distinct cables (no docks) provide a picture (of any description) on each display?

Grant Bennet-Alder wrote:

to be successful, a ThunderBolt Dock of any description must already Have the ports you need, like HDMI or DisplayPort, to connect your displays. Splitting a ThunderBolt out of your Mac into additional Thunderbolt ports does not help, because the split-off ThunderBolt ports do NOT have the ability to carry display data from the computer.

Other World Computing has Thunderbolt hubs and docks that do split Thunderbolt chains in a way that lets you plug USB-C (DisplayPort) displays or adapters into up to two of the downstream Thunderbolt ports.

For instance: Other World Computing – OWC Thunderbolt Hub

This is not the only such product on the OWC site, and if OWC can do it, there might be similar products on the SonnetTech and CalDigit sites (among others).

then we can talk about connecting two displays to one genuine ThunderBolt-3 or ThunderBolt-5 Dock.

What docks are you considering or attempting to use?