now:
  <xsl:apply-templates select="Documents/Document[Article/@info = 'main']"/>

  Would like to choose all article-elements where info="main" or info="sub".


since the Xpath for or is or, you more or less wrote the answer
yourself. I assume (based on your Xpath) that in fact you want to select
Document elements rather than Article elements, so something like:

Documents/Document[Article[@info = 'main' or @info='sub']]

David

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