Thanks Mathias for your quick reply.
If "Allow only a single instance of Multi Commander" is checked then if another instance is started the currently running will only pop to the front.
But if I check "Allow only a single instance...", there can surely only be one instance, not two. And sure enough, when I check this box, Listary does not open a new instance, it tries to open its folder in the
same instance of MC - only nothing happens.
If you want it to open a new tab with a specific path you need to tell the launching program to start MultiCommander with "MultiCommander /OPEN <Path>" then the <path> is sent to the other running MultiCommander and a new tab for that path is opened.
But I don't want to have another running MC, nor do I want an extra tab opening in the same pane in the same instance; I want to change the current tab in the current pane in the current instance, as happens in other Listary-compatible apps (Dopus, XYplorer, Total Commander, and of course Windows Explorer). It would be exactly the same as clicking an entry in MC's Favorites list.
Maybe I misunderstood you. I really hope that there is a way for an external application such as Listary (the best of them IMHO) to have this effect in MC.
If there isn't, I guess I will have to transfer my list of Favorites to MC's inbuilt Favorites feature; no big deal, but I have become accustomed to Listary. Overall, Multi Commander still seems the best Explorer replacement I have found yet, so a big thanks for your hard work here.