The problem
The Slack desktop app lets you add several workspaces, but they live in one window and you switch between them. You can't put work and a client workspace on two screens, keep one muted while the other notifies, or be signed in to the same workspace with two different accounts, for example an admin and a test user.
Opening Slack a second time just brings the existing window forward.
How to run a second Slack with Duplex
- Make sure you have Slack from slack.com. The Mac App Store version can't be duplicated. If you installed it from the App Store, delete it and download it from slack.com/downloads/mac.
- Install Duplex (instructions below).
- Click "New Instance…", pick
Slack.app, and name it, for example "Slack Client". Pick a badge icon so the two are easy to tell apart. - Launch it and sign in to your second account or workspace.
Now each Slack is its own app: its own window, its own Dock icon, its own notification settings, and its own login. Put them on separate screens or quit one at the end of the day.
What you should know
- Company sign-in. Email, password, and Google sign-in work inside an instance. If your company signs you in to Slack through Microsoft or with a passkey, that sign-in may not work inside the second copy.
- Notifications are per copy. The instance asks for notification permission once, separately from the original. You can mute one without touching the other.
- Updates follow the original. Keep Slack itself up to date. The instance rebuilds from the new version the next time it launches.
Get Duplex
Your first instance is free, forever. A $9 one-time license unlocks unlimited instances on up to 2 Macs. Requires macOS 13 or later.
brew tap bnimit/tap brew trust bnimit/tap brew install --cask duplex
Questions
Can I open two Slack windows for different workspaces?
Not with Slack alone: it shows one workspace at a time in one window. With a Duplex instance of Slack, each copy has its own window, so two workspaces can be visible at once.
Can I be logged in to the same Slack workspace with two accounts?
Yes. Each Duplex instance has its own login, so the original Slack and the instance can use two different accounts in the same workspace.
Does it work with Slack from the Mac App Store?
No. The App Store version checks its own identity and refuses to run as a copy. Download Slack from slack.com instead; it's the same app.
Does a second copy take up double the disk space?
No. Duplex makes a copy-on-write clone, which shares its disk blocks with the original. Finder shows the full app size, but a Claude instance costs about 6 MB of real disk.
Does Duplex change the original app?
No. The original is never modified and keeps updating itself. Each instance follows its updates automatically the next time you launch it.
Is Duplex free?
Your first instance is free, forever, which is all you need for a second account. A $9 one-time license unlocks unlimited instances on up to 2 Macs. There is no subscription.
Is it safe?
Duplex is native Swift, notarized by Apple, and collects no analytics. The source is public on GitHub, so you can read exactly what it does.