Dual Monitor Issue on MacBook Pro M4 Pro

Anker – Anker Prime TB5 Docking Station (14-in-1, 8K, Thunderbolt 5 )

That dock has four places where you can plug in a display:

• A HDMI 2.1 port
• A DisplayPort 2.1 port
• Two downstream Thunderbolt 5 ports

Anker says that the dock supports using two displays at the same time, and that you cannot use the HDMI 2.1 port and the DisplayPort 2.1 port at the same time.

You say that you tried a configuration where "Both monitors [were] connected to laptop Thunderbolt ports (using appropriate TB cables/adapters)" and it didn't work. Was that with, or without the Anker dock plugged in? That is, can you replicate the problem with the Anker dock taken completely out of the equation?

One reason I ask is that you have two monitors of the same type (27" Samsung Odyssey OLED G6 G60SD (QHD Resolution)). Monitors are supposed to have unique electronic serial numbers. But sometimes, a manufacturer might take a shortcut, and program a whole batch of monitors with the SAME serial number – to avoid having to write a slightly different firmware image for each one.

I can't say that this is the case here. But if you get two monitors from such a batch that do have the same serial number, that can confuse a computer that is relying upon those serial numbers to distinguish one from another.

If the problem happens without the Anker dock being plugged into the Mac at all, that would make me think we should (somehow) check into the possibility that non-unique electronic IDs might be the cause.

Samsung – 27" Odyssey OLED G6 (G60SD) QHD 360Hz 0.03ms FreeSync™ Premium Pro Gaming Monitor with Sleek Metal Design

This is a 27" monitor with a resolution of 2560x1440 pixels, and a maximum refresh rate of 360 Hz. The M4 Pro MacBook Pro does not support such a high refresh rate, at least not officially.

From Apple's Technical Specifications:

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M4 Pro

• Simultaneously supports full native resolution on the built-in display at 1 billion colors and:
• Up to two external displays with up to 6K resolution at 60Hz over Thunderbolt, or one external display with up to 6K resolution at 60Hz over Thunderbolt and one external display with up to 4K resolution at 144Hz over HDMI
• One external display supported at 8K resolution at 60Hz or one external display at 4K resolution at 240Hz over HDMI

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Back of the envelope calculation:

• UHD 4K @ 60 Hz = 497,664,000 pixels per second
• 2.5K @ 360 Hz = 1,327,104,000 pixels per second

so refreshing this monitor at 60 Hz would take about 2.7x as much bandwidth as refreshing a 4K monitor at 60 Hz. Hopefully the monitor is willing to run at a refresh rate that the Mac supports.

nikofromtornio wrote:

I have tried connecting monitors using laptop Thunderbolt ports and Anker dock was not plugged in, only one monitor shows image and other monitors says no input.

Those monitors have one DisplayPort v1.4 input, and two HDMI v2.1 inputs.

I wonder what would happen if you connected one monitor directly to the MacBook Pro using a HDMI cable and the other using a USB-C to DisplayPort adapter (cable).

I had M1 Mac Studio before this MacBook Pro laptop and i used same monitors with same cables without any problems, 2560x1440 at 120Hz and i used one Thunderbolt port per monitor

Mac Studio (2022) - Technical Specifications - Apple Support

"Simultaneously supports up to five displays:

• Support for up to four Pro Display XDRs (6K resolution at 60Hz and over a billion colors) over USB-C and one 4K display (4K resolution at 60Hz and over a billion colors) over HDMI"

There's no promise of any refresh rate greater than 60 Hz, and I can't find one in

Connect a display to Mac Studio - Apple Support

How many displays can be connected to Mac Studio - Apple Support

either. However, it seems possible that for resolutions lower than UHD 4K, faster refresh rates might be available, but unadvertised. Apple's Technical Specifications and Support documents don't have long tables of resolutions and refresh rates (like many monitor manuals do).