Hey, macOS seems to be designed to prefer single instances of applications indeed, but it’s possible to get around that by launching additional instances of the program using this command executed in a terminal:
open -n /Applications/Midihub\ Editor.app/Contents/MacOS/Midihub\ Editor
What will happen when there’s more than one device attached at a time - when you click Connect, the editor will open a list of all the available devices, the information displayed will also include the unique Serial Number of each device. You can pick which device to connect to from the list. Then you can launch the second instance of the editor, click Connect, it would show the list again, or if there’s only a single device without an active connection, it would just connect to it immediately.
The Editor instances can be differentiated from one another based on the Serial Number displayed at the bottom right.
That said, I did just give this a try on macOS, but there’s a bug - the 2nd instance of the editor interferes with the first instance’s connection and gets it disconnected. We’ll get that fixed.