I haven't seen any built-in support for something like that.  So you would  
most likely have to handle the key presses yourself, in whatever  
control/form is relevant for the input.  Upon receiving the first key  
press, you'll have to set some state (presumably also displaying some  
status to the user, such as changing text in a status bar or something)  
and then upon receiving the second key press, if that state is set, handle  
the key press as the completion for the command.

Pete