It's quite confusing how many external displays are usable with a MacBook that uses an M1 or M2 Chip. Documentation on the Internet is sparse and not even the Geniuses at the Genius Bar are 100% sure. We found out some things and can explain the backgrounds.
tl;dr: when using a dock for your MacBook, using more than one external display is only possible with workarounds.
Docking stations
Whenever you're considering using more than 1 external display, it makes sense to consider using a Docking Station for your Laptop of some sort. After all, you don't want to have to plug in 3 or more cables every day and connecting just the dock is way more comfortable. There are three main types of docks:
- pure USB-C - docks: They use DisplayPort over USB-C to connect to external displays, pass through USB peripherals and other ports such as RJ45 for network
- Thunderbolt - docks: They Use DisplayPort over USB-C to connect external displays, USB etc, same as pure USB-C docks, but with higher bandwidth (40 gbit/s vs. typically 10 gbit/s) and they can support downstream thunderbolt devices, such as special thunderbolt displays.
- DisplayLink - docks: They use DisplayLink to connect external displays to your laptop. This is supported by software running on your computer. This consumes computing power on the laptop and quality/responsiveness degrades if you're using many displays and/or high resolution
All of these docks most likely come with USB Type C plugs as the form factor.
Why only one display works and the others show the same picture
Most thunderbolt and USB-C docking stations make use of "DisplayPort Daisy chaining" (MST) under the hood. This means that if your dock has two or more ports (HDMI or DisplayPort) for external Displays, it's most likely not two display port signals but one daisy-chained ones. Computers as old as ThinkPad 570 from 2017 have no issue with this at all (we've tested). However, Apple Silicon based Macs don't support DisplayPort Daisy Chaining (MST). With most docks, you'll only be able to use one external display. The other screen(s) will just show the same picture as the first external display.
Workarounds
There is no solution to this problem available for the latest Macs. This is a hardware issue. There are only workarounds:
- Attach an HDMI cable to the second external display. This works but it defeats the purpose of a dock if you're connecting peripherals directly anyway.
- Use a DisplayLink dock. This comes with drawbacks in responsiveness and computing power
- Use thunderbolt screens. This comes with drawbacks in pricing, since thunderbolt screens are usually the professional type and reflect that in their price tag.
- Use an USB-C to HDMI-cable. This sounds weird but we made it work. If your docking station has a downstream USB-C port to connect other USB-C devices, you can use a cable that has HDMI on the other hand and connect your external display this way
Hardware we've tested this on
- MacBook Air M1: as expected, only one external display works
- MacBook Pro M1 with M1 Pro CPU: workarounds work
- MacBook Pro M2 with M2 Pro CPU: workarounds work
- Lenovo Thunderbolt 3 Gen2 dock: has 2 DisplayPort and 2 HDMI ports
- Lenovo Thunderbolt 4 Dock: has 2 DisplayPort and 1 HDMI port
- Lenovo Universal USB-C dock with DisplayLink
