[O] #+BIBLIOGRAPHY multiple entries

[yggdrasil]

I am trying to figure out how to use multipe bibliography files in
org-mode. What I've used so far is the #+BIBLIOGRAPHY statement which
lets me enter one .bib file, but I have been unable to use more than one
file. How would one go about to achieve this?

I use 
#+BIBLIOGRAPHY mybibliographyfile plain 
which works, but
#+BIBLIOGRAPHY myfirstbibliographyfile mysecondbibliographyfile plain 
does not.

Cheers,

Johnny