I don't have much experience with the two-pane outliner myBase (wjjsoft.com), but I thought it would be worth mentioning here that this year's version 7 of includes support for Mac (as well as Windows and Linux).

Thanks for this news, which I wouldn't otherwise have known about.

I've downloaded the application, and will play with it. (I've a vague memory of using it years ago, on Windows.)