The 'Comma Trick' allows a multiple selection in the first pane of Quicksilver (QS).
A comma is the character between the following quotes: ','
To select items from within the same folder:
Invoke QS, navigate to within a folder, press comma, and navigate to/search for another item in that folder.
To select items from different folders:
Invoke QS, navigate to within a folder, press comma, press escape, and search for another folder.
Or:
Invoke QS, navigate to within a folder, press comma, and press left or right to navigate to another folder.
To select items from anywhere in the QS Catalog:
Invoke QS, search for a cataloged item, press comma, and search for another cataloged item.
The last selected item doesn't require a comma press. (In the above picture, System Preferences.)
Pressing delete will remove the last comma-selected item from the selection. (In the above picture, TextEdit.)