You bought a USB-C KVM so a Mac and a second computer could share two monitors. Both screens light up. Both show the same image. The cables are seated. Display Settings looks normal. The Mac isn't broken.
This is usually MST. The switch is sending one display stream and splitting it. Windows and Linux can turn that split into two extended desktops. macOS cannot, so both monitors clone.
The rule is simple: count the video streams the Mac is actually sending, not the number of ports on the back of the switch. If those two outputs are MST, no setting, cable, or M-series upgrade will turn them into two desktops.
What mirroring actually is
Mirroring means both external monitors show the same desktop. One window, two copies. Dragging a window toward the "second" screen does nothing useful because there is no second desktop.
That's different from a single-output KVM, which only has one monitor port. It's also different from a Mac that can drive only one external display natively. Those are chip limits. Mirroring through a dual-output USB-C KVM is a topology limit. The switch is asking macOS to split one DisplayPort stream, and macOS won't do that.
If System Settings > Displays lists two external displays as duplicates, you're looking at this problem. Changing arrangement, resolution, or Extend won't create a second desktop if the switch is MST. This page is not a dual-monitor cable walkthrough. The question here is why two KVM outputs still clone on a Mac.
MST vs native dual-extend
A KVM can drive two monitors from USB-C in two ways.
MST (Multi-Stream Transport) takes one DisplayPort signal and splits it into two. One host cable in, two monitor cables out. Windows and Linux treat the split as two independent displays. macOS doesn't support MST over USB-C, so both outputs show the same picture.
Native dual-extend means the computer sends two real display streams. The switch routes stream one to monitor one and stream two to monitor two. That's what extended dual actually is.
A switch can have two HDMI ports and still be MST. The holes on the box are not the streams.
The Cable Matters 201085 is the MST case that catches Mac buyers. Each laptop uses one USB-C cable with DisplayPort Alt Mode. The switch drives separate HDMI and DisplayPort outputs. On Windows or Linux those outputs extend at up to 4K@60. On a Mac they mirror. The switch doesn't charge the laptops (the Micro-B port only powers the unit) and it has no EDID emulation. Cable Matters lists it for laptops, not desktop PCs.
The AV Access iDock C20 uses the same MST idea for two USB-C laptops: dual 4K@60 HDMI, up to 60W charging to each host. A connected Mac can use the dock. The two external screens still duplicate.
The two-cable native path is the AV Access iDock M10. The MacBook connects through two USB-C ports so the dock can receive two native display streams. The second host is a desktop on HDMI, DisplayPort, and USB, not a second laptop on one USB-C cable. Both hosts then share two HDMI outputs rated for dual 4K@60, and the MacBook can take up to 100W Power Delivery. On a Mac that already drives two external displays natively, the M10 can route both as extended desktops. It cannot make a two-laptop desk work, and it cannot add a second native stream to a base M1 or M2 MacBook.
Why this happens on every Mac, including M4 Pro and Max
People hear that base M1 and M2 chips drive only one external display and stop there. That's a real Apple Silicon limit, and it's a different problem.
The Cable Matters 201085 mirrors on every Mac, including M4 Pro and M4 Max, because the switch is MST. A chip that can drive two or four native displays still sends one DisplayPort stream into that KVM. macOS won't split it.
Keep the two limits separate:
- Chip limit: Base M1 and M2 output one native external display. M3 base MacBooks can do two only with the lid closed. M4 and M5 base models, plus Pro and Max chips, can send two or more native streams.
- Switch limit: An MST KVM asks the OS to split one stream. macOS doesn't. The Mac's native display count doesn't change that.
Buying an M4 Pro or Max doesn't give you dual-extend on the 201085 or the C20. Those switches still mirror.
If the real constraint is the chip (base M1 or M2, or an M3 base MacBook that must stay open), that's a native-output limit, not MST. The USB-C KVM guide for Mac and Windows covers that path, including DisplayLink, separately.
One 4K@60 is the USB-C default
Generic USB-C at 10Gbps carries one 4K@60 display, not two. That's the bandwidth default, not a Mac quirk.
Dual 4K@60 on USB-C is one of two things:
- MST. Windows and Linux extend. macOS mirrors.
- A two-cable native path, such as the iDock M10 for a MacBook plus a desktop.
There is no generic single-cable USB-C KVM that gives a Mac two extended 4K@60 screens and also switches two laptops. If a listing promises dual 4K USB-C without saying MST or naming a two-cable native layout, treat it as MST until the topology is proven.
A Thunderbolt label on the box doesn't change this by itself. USB-C and Thunderbolt share a connector. The protocol and the switch topology still decide whether you get one stream or two.
What will not fix it
A few common next steps don't change the picture.
Display Settings. Setting Extend on an MST Mac path doesn't create a second desktop. The OS is still looking at one stream.
New cables or a faster USB-C port. If the switch is MST, a Thunderbolt 5 port on an M4 Max doesn't turn that switch into two native streams.
An M-series upgrade for the 201085. The 201085 mirrors on M4 Pro and M4 Max for the same MST reason it mirrors on every other Mac.
A 4K@240 gaming monitor through the TESmart. The TESmart HDC202-X24 cannot do 4K@240. It maxes at 4K@60 HDMI 2.0, 1440p@120, or 1080p@240. Its two monitor outputs are HDMI 2.0. That's a refresh-rate ceiling, not a Mac Display Settings issue.
Treating the TESmart as a confirmed Mac dual-extend fix. Dual-extend on Apple Silicon is unconfirmed. TESmart's product FAQ says ARM Macs only support DP SST, so both HDMI outputs duplicate. TESmart's marketing blog claims native dual-extend on Pro and Max chips. Those two manufacturer sources conflict. We have no hands-on. Don't buy the HDC202-X24 expecting extended dual displays on a Mac.
The HDC202-X24 is a laptop-plus-desktop Thunderbolt 4 switch: Thunderbolt 4 USB-C on the laptop side with 60W charging, DisplayPort plus HDMI on the desktop side, and two HDMI outputs. Use it when you need that Thunderbolt link. Don't use it as the Mac dual-extend answer.
The M10 is also not a fix if your second computer is a laptop. It needs the desktop input group. Pointing a two-laptop Mac desk at the M10, the C20, the 201085, or the HDC202-X24 won't produce confirmed extended dual displays on macOS.
Where to go next
If you still need to pick hardware, use the two ranked pages. This page doesn't add a new list.
For USB-C host pairings, MST vs native paths, charging, and the DisplayLink workaround for chip-limited Macs, read the USB-C KVM guide for Mac and Windows.
For the Thunderbolt 4 switch, the 201085 MST caveat, and TESmart's refresh and Mac-display limits, read the Thunderbolt KVM guide for Mac and Windows.
If both monitors already show the same image on a Mac, start with topology. If the switch is MST, those two outputs will stay clones.