How can I use two external monitors with my MacBook M4?

Aydown wrote:

How can I use two external monitors on my MacBook with M4? My M4 only recognizes one of the two monitors. Both monitors are connected to an Icybox docking station. One via HDMI to mini-DP and the other via HDMI to USB-C. Do the monitors have to be connected to a dock via Thunderbolt?

The dock, hub, or dual-display adapter connected to the Mac has to be Thunderbolt.

The monitors connected to that device don't.

Looking on the IcyBox site, I saw lots of USB-C docks/hubs, one USB4 hub, and only one Thunderbolt hub. So I'm guessing you have a USB-C docking station. If it's using hardware video output, it will support only a single display when attached to a Mac. If it is using some workaround, it might support more with installation of a driver, but only in a second-class way, with compromises.

Aydown wrote:

@Grant Bennet-Alder

But my is a Thunderbolt Dock.

https://www.digitec.ch/de/s1/product/icy-box-thunderbolt-3-triple-dock-85w-pd-usb-a-thunderbolt-usb-c-15-ports-dockingstation-usb-hub-10244263?shid=1890225

It also has four places to connect monitors and supports connecting three displays.

The relevant line from the dock's description (automated translation): "4 video interfaces – triple video output! Mini DP 1.2, VGA and HDMI 2.0 or DisplayPort 1.2

Macs only support connecting two displays to a Thunderbolt dock.

I suspect that this dock Is getting the extra outputs by using the equivalent of DisplayPort MST daisy-chaining – which Macs do not support. So the dock is taking two video signals from the Mac, and duplicating them to get three or four outputs. Choose the wrong pair of outputs and they will both be running off a single video signal – so the monitors will "mirror" each other and the Mac may not even be aware that there are two monitors.

I've seen this with HP and Dell Thunderbolt docks that have more than two places to attach displays.

Maybe if you look in the dock's manual, or play around with using different combinations of two outputs, you can find a combination of outputs that will work.

Grant Bennet-Alder wrote:

I found a rudimentary manual here:

PDF Manual IB-DK2801-TB3 - Amazon Web Services

The Mac part of the "Video Output Table" seems to indicate that to get dual display output on a Mac, you would connect

• One display to the DisplayPort, HDMI, Mini DisplayPort, or VGA port
• One display to the Thunderbolt 3 daisy-chaining port (perhaps with the aid of a USB-C to (whatever) adapter)

To the OP: When you say that you are connecting monitors in this way:

"One via HDMI to mini-DP and the other via HDMI to USB-C"

What is the direction in which that "HDMI to mini-DP" adapter is meant to be used? Many display adapters are unidirectional. Typically Mini-DP or USB-C would be for the source, and HDMI for the output (monitor).

The Mac only allows ONE display on a port, except when the Port, the Cable, and the first device (Dock or Display) are all genuine Thunderbolt. USB-C has far less capacity due to its lower speed, so it will not suffice.

Windows has no standard for what is acceptable video quality, so your Dock may work fine on Windows.

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My recommendation would be connect one display on a DIRECT HDMI cable connection (no dock) and the other to your existing Dock.

Cable specs and cable lengths can also be an issue for successful operation of high-end displays on a Mac. For example, the HDMI cables that came for behind your TV set are likely inadequate for high-end displays.

To use up to TWO displays, they must be limited in resolution so that the combined data rate of ThunderBolt-3 is not exceeded.

Also, your Host cable MUST be a genuine ThunderBolt-3 or ThunderBolt-4 cable no longer than 0.5 meters, regardless of what cable was included in the box.

the host port appears to be port 5. Be sure that is the one where you are connecting your host cable.

I found a rudimentary manual here:

PDF Manual IB-DK2801-TB3 - Amazon Web Services

Then it needs to be a Thunderbolt dock, not the one you have.