MacOS: open -a Amaya.app a b will only open a

I attempted to use open to open some HTML files in Amaya:

  $ open -a /Applications/Amaya.app/ xmlsec-charter.html
    ../../2005/Security/xmlsig-charter.html 

Only the first of the files I passed was opened.

Expected behavior: Open both.

Thanks,
-- 
Thomas Roessler, W3C  <tlr@w3.org>