Second monitor is set to mirror rather than Extended display

There are two problems.

One is that the 15" M2 MacBook Air – a machine which Apple discontinued in March 2024 – supports only one external display. If you have misidentified the machine and it is actually a 15" M3 MacBook Air, it supports one external display with the lid open, two with the lid closed.

MacBook Air (15-inch, M2, 2023) - Tech Specs - Apple Support

MacBook Air (15-inch, M3, 2024) - Tech Specs - Apple Support

Use dual monitors with your MacBook Air and MacBook Pro with M3 chip - Apple Support

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The other is that the Belkin Connect USB-C 11-in-1 Multiport Dock is not a Thunderbolt dock.

Macs do not support DisplayPort MST daisy-chaining or its equivalent. They support connecting one display to "plain" USB-C docks; up to two displays to Thunderbolt docks.

Belkin's dock pretty clearly is not a Thunderbolt dock, and Belkin's description of their dock says that it will only support mirrored displays on macOS. I would guess that the Mac itself only sees one of the monitors and that it would be the dock itself taking the single signal that the Mac provides, and distributing it to both monitors.

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So, which chip (M2 or M3) does your 15" MacBook Air have?

Hi,

In your settings you should be able to select you want to mirror or extend your MacBook display. Please take a look for steps from article below:

Extend or mirror your Mac desktop across multiple displays - Apple Support

Please, check at first there is configured correctly.

Jusch2 wrote:

M2

Then your MacBook Air supports only a single external display.

Unfortunately, as

@Servant of Cats mentioned, according to product specification dock supports only screen mirroring with Macs… Weird

https://www.belkin.com/p/usb-c-11-in-1-multiport-dock/P-INC004.html

There are workarounds to attach more displays than your computer's hardware supports, but these can involve significant compromises.

They involve getting an external "stunt box", equipped with a special chip set, and installing the matching driver software on your Mac. DisplayLink, by Synaptics, is one of the more common workaround technologies. (Note that despite the similar names, DisplayLink is not the same thing as DisplayPort.)

If you use a DisplayLInk-based workaround, it is quite likely that you'll need to go to the Synaptics site to get the matching driver, and/or to consult Synaptics' support articles.

Synaptics – DisplayLink

DisplayLink Support – Knowledge Base – Troubleshooting: macOS

Can you please send screen from your Mac Display settings with connected display?

If you connect your display to MacBook without Dock, everything works? "Second monitor" means you connect 2 external displays or 1 additional to your Mac?

Whats’s happen when You open „Use as” „Main display” You see different options?