Using a single Rake dependency block to generate more than one file.

Often I have a block that generates more than one file.   Although  
Rake seems to accept reasonable syntax for doing this it doesn't  
work.  Here's a complete but simple example:

require 'rake'

a = 'file1.txt'
b = 'file2.txt'
file [a, b] do
   `touch #{a}`
   `touch #{b}`
end

task :all => [a, b]


Although Rake excepts this syntax, it doesn't do what I'd expect.  It  
gives me the surprising answer that it doesn't know how to build  
file1.txt even though it really should know.   I realize I could copy  
the block or define a function for it and use multiple file  
dependency declarations but this seems really clumsy and not very  
DRY.  Also, while such a workaround appears to work on my platform  
there isn't any reason to believe that it shouldn't call those blocks  
twice.  The fact that it doesn't makes me think Rake is checking file  
creation too often, i.e., before it could reasonably expect a file  
exists.  Am I correct about all this?

Joe