You're right, if done so by purpose, but I thought that this only concerns the macOS dialogues, in which I want to navigate by keyboard.
All other apps I've used till now with two panes (Forklift, Pathfinder, and so on) are using the tab-key for jumping from one pane to the other.
I would greatly appreciate if Yummi FTP allows user defined shortkeys in future versions, the tab-key function included
Thank you very much for reading and for your help.