I thought the DE Coaches wanting to use this might find the following helpful;

This Guide assumes you have rolled a double on level up, and have selected Multiple Block from the Strength tree.

• Select the Assassin
  
• Left click on him to bring up his actions list (blitz, multi-block and stab) and select multi-block
  
• Left click him again and select stab
  
• Left click on target (that's the first stabbee)
  
• Left click Assassin again, bring up the list - again - and select stab - again
  
• Left click on second target to stab a second person / thing.

That's Assassin; Multi-Block; Stab; Target; Assassin; Stab; Target. Phew!

Thanks to MercyFlush over at the BB Official Forums[forum.bloodbowl-game.com]for this (though his steps 4 & 6 didn't seem to work for me - I'd L Click instead of R).

YouTube video to follow ...