Taxonomy Menu: Display Descendants does not work
With taxonomy menu 2.0alpha2 if you select multiple you'll see the warning : "Multi-terms path is not available in Drupal by default, which means that you will have to register it using a module like Views for example."

Luckily views provide by default a taxonomy term view to emulate drupal default taxonomy term handling, so you need to activate it. 
But even with this view activated you wont see your menu items, and this is because the path generated by taxonomy menu(ie: taxonomy/term/tid1+tid2 ) is still not handled.

This is because default taxonomy term view handle multiple terms ids argument but does not validate them (WTF !?).

So what you need to do in a nutshell:


  
  In taxonomy menu 2.0alpha2 module, select "Menu path type => Multiple terms"
  Activate taxonomy term view
  In [ Contextual filters =>  Content: Has taxonomy term ID (with depth) => Specify validation criteria => Filter value type ] change to "term IDs separated by , or +"
  


There you go !

ps : I did not try this with taxonomy menu 1.4, anyone ?