This has been a problem since AC2 (perhaps even AC1 - I forgot?). Why hasn't this been fixed? Since 2009, fans have asked for simultaneous attacks. At the 2010 E3 demonstration of Brotherhood, developers claimed guards would be now allowed to attack more than one a time, but obviously that was not the case. To this day, guards still form a circle around you, leaving you to wait until one attacks so you can counter and begin a chain.
There used to be so many threads about this around the launch of ACB, but no one seems to specifically address this anymore. It's not like it's been solved. Four years of development and the difficulty of AC combat has hardly evolved. Please, revamp the combat system for AC3, Ubisoft.