Connecting two laptops with an HDMI cable won’t give you an extra screen. On most laptops the HDMI port only outputs video, so neither computer will accept the other as a display. There are still ways to do it, and the right one depends on the operating systems involved and how much lag you’re willing to accept.
You get two layouts with a second screen. Mirroring puts the same desktop on both displays, which helps with presentations. Extending stretches one desktop across both, so you can see more tabs and juggle more tasks at once. With an unused laptop in a drawer and a few settings tweaks, you can skip buying a new monitor.
Windows to Windows: Miracast comes built in
When your main computer and the spare laptop both run Windows 10 or 11, you already have the casting tools. They rely on Miracast, which links devices over Wi-Fi.
Begin with the spare laptop. Go to Settings, then System, then Projecting to this PC. Select Optional features, then View features, and add Wireless Display. Once the install is done, return to that page, choose Available everywhere and launch the Wireless Display app.
Next, on the main computer, press Windows key + K and select the spare laptop from the available displays. Type in the PIN and permit keyboard and mouse input. After that, decide whether to duplicate or extend the screen. You can flip between the two whenever you like with Windows key + P.
The drawback: both computers need Wi-Fi, and some lag is possible. A presentation or a screen full of browser tabs will cope with that, but gaming may not.
Mac to Mac: AirPlay, for 2018 models and newer
AirPlay can link two Macs, as long as the MacBook serving as the second screen dates from 2018 or later. Anything older won’t work.
On the MacBook you’ll use as the second screen, go to System Settings, pick General, then AirDrop & Continuity. Switch on AirPlay Receiver, and set Allow AirPlay for to Current User. Log both MacBooks into the same Apple ID so they connect without trouble.
Then, on the main Mac, go to System Settings, then Displays, and click the plus sign in the dropdown menu to pick the second MacBook from the list. You choose mirroring or an extended display in settings. Like Miracast, AirPlay works over Wi-Fi, so expect a small delay.
Windows, Mac and Linux together need extra software
Pairing a Windows PC with a MacBook, or using Linux anywhere in the setup, calls for a third-party tool. Deskreen is one option: it’s free, open-source and runs over Wi-Fi.
Connect both computers to the same Wi-Fi network, then install Deskreen on the main one. It generates a QR code and a URL. Load either one in the spare laptop’s browser to link the two machines.
Out of the box, Deskreen only mirrors. If you want to extend your desktop to the spare laptop, insert an inexpensive virtual display adaptor (known as a “dummy plug”) into the HDMI port on your main computer. This is also a Wi-Fi link, so lag comes with it.
Going wired with a capture card
To cut down on lag, use a cable and an HDMI-to-USB video capture card. Connect the HDMI cable from your main computer to the card, then plug the card’s USB end into the spare laptop.
To the spare laptop, the capture card looks like a webcam. You can view the feed in its camera app or in OBS Studio, a free video app that runs on Windows, Mac and Linux.
Consoles work with this setup too. You can hook a PlayStation or Nintendo Switch up to the laptop the same way and play without a TV. Some lag remains, though it’s usually lower than with the Wi-Fi options, and it probably won’t matter unless you play competitively.
And if a second screen isn’t something you need, the old laptop can be turned into a smart home hub instead.