Plan · 21.10.2026

MacBook Pro as external display

The 16-inch MacBook Pro gets a second screen: the 14-inch MacBook Pro beside it. One USB-C dummy plug makes macOS see a real second monitor. One Thunderbolt cable carries its picture across, and a Swift app shows it full screen.

Shopping list Dummy plug + Thunderbolt 4 cable: €48.93
The 16-inch MacBook Pro with a USB-C dummy plug, joined by a glowing Thunderbolt 4 cable to the 14-inch MacBook Pro, which shows its second desktop; the BenQ stands behind the 14-inch.
The whole system. Drawn with Nano Banana Pro in Gemini.

1Shopping list

Checked on amazon.de on 21.10.2026: the resolution tables on the product pages, the prices and the stock. Prices change, so each link shows today's price.

USB-C dummy display plug
Must have

USB-C dummy display plug (EDID emulator)

Goes into a USB-C port on the 16-inch. Its table lists 2880 × 1800 @ 60 Hz and 2560 × 1600 @ 60 Hz. Both are 16:10, the same shape as the 14-inch screen under its notch (3024 × 1890), so there are no black bars.

€14.28
Open on Amazon.de › B0FCDJG6SW
Cable Matters Thunderbolt 4 cable
Must have

Cable Matters Thunderbolt 4 cable, 0.8 m, Intel-certified

40 Gbps. Plugged between the two Macs, it becomes the Thunderbolt Bridge, a private network with almost no delay. It must be a Thunderbolt cable: a USB-C charging cable makes no network.

€34.65
Open on Amazon.de › B08KSM2FWR
USB4 40 Gbps cable
Cheaper choice

USB4 40 Gbps cable, 0.8 m, USB-IF certified

Instead of the Cable Matters cable. USB4 at 40 Gbps carries Thunderbolt between Apple-silicon Macs, but it has no Intel certification. If the bridge does not appear, this is the first suspect.

€13.65
Open on Amazon.de › B09LV3S9DH
BENFEI USB-C to HDMI adapter
Only if needed

BENFEI USB-C to HDMI adapter, 4K60

Not needed for the plan. It is only for keeping the Nano plugged in when the 16-inch's own HDMI port is busy.

€7.34
Open on Amazon.de › B08KWG243P

2Which port gets what

The 16-inch and the 14-inch, each seen from both sides.

MacBook Pro 16" M1 Pro · the sender Left side MagSafe: power Thunderbolt cable → 14" USB-C dummy plug Right side HDMI → CineView Nano (step 1) Thunderbolt: free SD card MacBook Pro 14" M1 Pro · the screen Left side MagSafe: power Thunderbolt cable ← 16" Thunderbolt: free Right side HDMI → BenQ (stays) Thunderbolt: free An M1 Pro drives two external screens. On the 16-inch they are the dummy plug and, in step 1, the Nano on HDMI.

3The signal path

From the dummy plug's screen on the 16-inch to the full screen on the 14-inch. All of it is in Swift.

Six steps: dummy plug, ScreenCaptureKit, VideoToolbox HEVC encode, Thunderbolt Bridge, hardware decode, full screen at 2880 by 1800.

4Step 1 needs nothing new: the CineView Nano

Before anything is bought, the Nano you own plays the dummy plug. The 16-inch's HDMI goes into the Nano, and CINEVIEW_VIEWER on the 14-inch shows the picture over the Nano's Wi-Fi. This proves the receiver first.

The 16-inch MacBook Pro's HDMI into the Accsoon CineView Nano, whose Wi-Fi reaches the 14-inch MacBook Pro running CINEVIEW_VIEWER.

5The holes, and how each is closed

Two external screensThe M1 Pro drives two, which covers the dummy plus the Nano. A plain M1 drives only one, so the sender must be the M1 Pro.
The dummy's resolutionThe plug lists 2880 × 1800 and 2560 × 1600 at 60 Hz, both 16:10 like the 14-inch. A cheap plug with only 2560 × 1600 at 30 Hz was left out.
The cableThunderbolt with the lightning mark, not FireWire, and not a charging cable. The bridge appears by itself, and the two apps find each other through Bonjour.
Who "registers" the displayThe dummy registers on the 16-inch. The 14-inch only shows a full-screen window, mouse pointer included.
The Accsoon formatIt sends packets with no resend. On a cable almost nothing is lost, and one receiver then shows both the Mac and the Nano.
Never run yetCINEVIEW_VIEWER has never been compiled or run. Step 1 is its first run.
Sharpness2880 × 1800 is shown at 3024 × 1890, which is 5 % larger and nearly sharp. On the day we try BetterDisplay for the exact size.

6The order of work

  1. The Nano testThe 16-inch HDMI goes to the Nano, and CINEVIEW_VIEWER runs on the 14-inch. Nothing to buy.
  2. Buy the two partsThe dummy plug and the Thunderbolt 4 cable, from the shopping list.
  3. Mantra Display sender v1On the 16-inch: grab the dummy's screen, encode it in hardware and send it in the Accsoon format over the cable. The receiver is CINEVIEW_VIEWER.
  4. v2 and laterv2: the full native size in HEVC. v3: the 14-inch's keyboard and trackpad drive the 16-inch. v4: Wi-Fi instead of the cable.