Mimiya, I'd never used the File browser that way, but just to check how it would work, I selected two image files and both opened successfully, one after the other. So try restoring your preferences to their defaults. Here's Stephen Marsh's instructions for that process:
restore preferences to defaults
How much memory does your computer have? It takes a good deal of memory to open two or more large files. What happens when you use the menu to open more than one file: (File-->Open ) -- any differences?