I'm not averse to a command line or script solution to launch the second app, obviously. I can use a web client for one account, of course. But they are different.

Sad that I can edit my post to clarify things.

I don't know if this would meet your needs, but I believe you could have different accounts open at the same time in different web browsers.

Hi jrrt,
Thanks for your patience in responding over the holidays. We're catching up this week!

We are not aware of a way to run two instances of the desktop app manually, at least in the same user account. (You could switch between multiple macOS users, but I expect this may not be what you want.)

As geojono mentioned, you can juggle multiple accounts between the desktop app and web app. Between private/incognito windows of a browser and multiple browsers, this is what the Remember The Milk team does when developing and testing, since we can open many windows at once. (Then the only problem is keeping them straight! 😅)

We hope this helps!