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How to run two instances of the same app on Mac
“Run two instances” usually means one of two different things: two windows of one account, or two fully separate accounts running at the same time. Here's every real way to get either, and which is which.
Two windows vs. two accounts — decide which one you need first
If you just want a second window of the app you're already signed into — a second Slack window pinned on another desktop, say — most apps already support that from their own Window menu, no special approach required. What actually gets asked here is almost always the harder version: a genuinely separate account, signed in independently, open at the same time as the first, without logging one out to use the other.
The five approaches below solve that second problem to different degrees. None is universally best — the right one depends on how often you switch and how separate you actually need the data to be.
Terminal: open -n
open -n -a "AppName" tells macOS to launch a fresh process instead of activating one that's already running. For apps that don't defend against it, this genuinely gives you a second process — but not a second account, because that process still reads the same on-disk data directory as the first. A large share of apps refuse outright, too: Electron apps in particular commonly lock against a second instance, so the new process just forwards its launch to the first and exits. Full mechanism in the dedicated guide.
Best for: apps that don't single-instance-lock, when two windows of one account is genuinely what you wanted.
A second macOS user account
System Settings → Users & Groups → Add Account gives you a completely separate home directory — the most thorough isolation macOS offers, because it isn't a workaround, it's what user accounts are for. The cost is that it's a full login: Fast User Switching keeps both sessions running, but switching is a slower, heavier context switch than ⌘Tab, and you can't see both accounts' windows on screen at once.
Best for: a hard boundary you rarely cross — a shared family Mac, or a machine used for one employer's work and nothing else.
Browser profiles
If the account you need a second of lives on the web — Gmail, most SaaS tools, anything you'd normally reach at a URL — browser profiles are the cheapest real answer. Chrome, Firefox and Edge all support adding a second profile with its own cookies, extensions and login, open in separate windows at the same time. This only covers things you access through the browser: a native desktop app built around the same service doesn't inherit browser profile switching, because it isn't a browser tab.
Best for: web-based accounts, when you don't specifically need the native desktop app.
A virtual machine
Parallels or UTM gives you an entire second macOS instance, which trivially solves the isolation problem because it's a different computer as far as any app is concerned. It's real, and it's also the heaviest option by far — a meaningful chunk of RAM and disk set aside permanently, and a full OS to keep updated, usually for a single app's second login.
Best for: isolating more than one app at once, or an OS version the host can't run natively.
A real second copy, with its own identity
The direct fix for the actual mechanism — see why a Finder copy doesn't work — is an app copy carrying a genuinely new bundle identifier, its own data directory, and a valid re-signature. Done right, it's the only approach here that gives two simultaneously open, independently signed-in copies of a native app, each with its own Dock icon, without switching users or running a second OS. Building that by hand is real, tedious work per app; tools like MacDupl automate it in a couple of seconds.
Best for: apps you switch between constantly and want both open permanently, without ever logging out.
Compare them side by side
| Approach | Isolation | Both open at once? | Setup |
|---|---|---|---|
open -n | None — same account, if it opens a window at all | Only if the app allows it | One Terminal command |
| Second macOS user | Complete — separate home directory | One session visible at a time | A few minutes, once |
| Browser profile | Complete, for web accounts only | Yes, side by side | About a minute, once |
| Virtual machine | Complete — a separate OS | Yes | An hour+, real ongoing resource cost |
| A real second copy | Complete — new identity, own data | Yes, side by side, both signed in | Manual: hours per app. Automated: seconds |
Questions
Which of these lets me see both accounts on screen at the same time?
Browser profiles, a virtual machine, and a real second copy all do. A second macOS user technically runs both sessions but only shows one on screen at a time; open -n usually doesn't give you a second account to see at all.
Is open -n ever the right answer?
Yes, for apps that don't enforce single-instance and where two windows of the same signed-in account is genuinely what you wanted.
What does MacDupl automate specifically?
The last row — generating a new bundle identifier, a private data directory and a valid signature for a real second copy of an installed app, so it launches as its own independent app in seconds instead of by hand.
Try it on your own Mac
The free trial classifies every app in your Applications folder instantly, so you can see exactly how a specific app is handled before paying anything.