One YouTube broadcast, horizontal and vertical

TL;DR

One broadcast can reach Shorts-feed viewers tall and everyone else wide. It is on by default, with a center crop for the tall version.
A center crop of our desktop keeps about a third of the width: fine for a face, hard on code. Encoder mode takes a second key and a vertical feed you compose.
The tall feed has no clickable links, no live ads and no 4K on the app. Our stream ran as two broadcasts, not one, and I did not run Dual stream itself.

Step 1: Know what you are turning on

I re-read YouTube’s live streaming Help page on Oct 2, 2026. Streams started from the Live Control Room go out in two shapes, 16:9 and 9:16. Viewers in the Shorts feed on the mobile app get the tall one. Everyone else gets the wide one.

The page says it is the default, and a Dual stream switch under Stream settings turns it off. Our notes describe switching it on, so something changed or they were loose. I read this feature and did not run it. Our own run was different (Step 5).

One rule sets your order of work: You can’t add the vertical format once the stream has started. Decide before you click Go live.

Step 2: Run the crop arithmetic on your own canvas

The Help page is plain about the default: The vertical stream is a center crop of the horizontal stream, with other vertical layout options available below.

Our desktop is 16:10, and on Aug 29, 2026 the broadcast canvas was set to match, so capture is one to one. A 9:16 slice at full height is 675 pixels wide, about 35 percent of the frame:

frame   1920 x 1200
kept     675 x 1200   (1200 x 9 / 16)

[ cut off | kept 675 | cut off ]

A face survives that. A code editor loses its gutter, its file tree and the ends of long lines. This is arithmetic: the crop has never been aired.

Step 3: Choose Auto or Encoder

Auto is free. Encoder costs you a second feed.

Auto versus Encoder. Source: YouTube Help, re-checked Oct 2, 2026. The last two rows are our judgment and our own run.
Question	Auto	Encoder
Stream keys	One. YouTube makes the tall version.	Two. The second key carries your vertical feed.
Who frames the tall view	YouTube. Center crop by default. Fit to phone width, Stacked, zoom and position under Edit layout.	You, in your own broadcast software.
What it costs you	Nothing extra.	A second encode, more upload, and a second scene to maintain. Our second feed was set to 6 Mbps beside 8 Mbps.
Where I would use it	Face-to-camera talk, where the middle of the frame is the subject.	Screens and gameplay, once the free layouts and the zoom control fail a test on your real content.

Step 4: Compose the vertical feed yourself

Our vertical setup was 1080x1920, run as its own profile and scene collection: the webcam in a band on top, the whole desktop fitted below it at 1080x675, uncropped, the rest empty. I want every pixel of the screen visible.

Visible is not readable. A desktop 1920 pixels wide, fitted into the tall frame and shown about 390 pixels wide on a phone, lands at about a fifth of its size, so 14-pixel editor text comes out near 3 pixels. The center crop keeps text about 2.8 times larger, with less of the screen. That is arithmetic, not a test.

Our vertical profile was set to 6 Mbps beside the 8 Mbps horizontal feed, 14 Mbps combined. Our notes plan for 21 to 28 Mbps of upload, 1.5 to 2 times that: a rule of thumb, not a YouTube number.

Step 5: Switch to Encoder and go live

The Help page lists the path. In YouTube Studio:

Open Create, then Go live, then the Live Control Room, then the Stream tab.
On the vertical preview, open the Auto dropdown and choose Encoder.
Pick the RTMP(S) protocol.
Open the Stream key dropdown and choose your second key.
Point your vertical (9:16) output at the second key and your horizontal (16:9) output at the first. Check that both feeds carry matching content in the right format for each key, as the page asks.
Go live from Studio.

The limits on the same page: 10 active streams per channel and 3 per stream key, applied at the same time.

Step 6: Plan around what the tall feed lacks

The Help page’s comparison table lists what the vertical feed lacks: clickable links in chat and in the channel description, live ad midrolls and pre-rolls, 4K bitrate on the YouTube app, premieres and live redirect. Memberships work in both, but a second Help page says buying one in the vertical feed is not currently available on iPhones.

That page also says the vertical feed is only on the app, not currently on tablets, and does not show scheduled streams or Premieres. Put links in the horizontal description.

Run it yourself

Compute the 9:16 slice of your canvas.
Try each layout, zoom and position in Edit layout on your real content.
Decide Auto or Encoder before you go live. You cannot add the vertical format later.
If Encoder: check whether your software or a plugin you already run offers a vertical canvas (update it first), then build the vertical scene and check the key it sends to.
Compare combined bitrate with your upload speed, with headroom.
Move links to the horizontal description.
Plan the Short you will cut from the horizontal replay.

Sources

YouTube Help: Get started with live streaming (opens in a new tab), re-checked Oct 2, 2026. Supports: the default, toggle, center-crop sentence, layouts, Encoder steps, limits, feature table, and no vertical format once started.
YouTube Help: Watch and interact with vertical live streams on the YouTube app (opens in a new tab), re-checked Oct 2, 2026. Supports: app only, tablets, iPhone memberships, scheduled streams.
YouTube Official Blog: Announcing our biggest updates to YouTube Live (opens in a new tab), Sep 16, 2025, read Oct 2, 2026. Supports: the announcement date and the word soon.
PPC Land, trade press (opens in a new tab), Aug 21, 2026, read Oct 2, 2026. Not official. Used only for the Aug 20, 2026 encoder date and one report about the vertical replay.
Our own check of the channel’s public stream entries, read Oct 3, 2026 (see the archive guide).
Our own notes (Aug 29 and Sep 7, 2026; the Dec 2025 plugin build), not re-verified against current docs. The Step 4 figures are pixel arithmetic, not a test.