Your hero got lucky! Yes he can use both cards at the same time and only misses on an "X" (1 out of 6 attacks).
If this is too much for the Overlord to handle you could implement a house rule stating you can only use one skill per attack.
Hm, this can be easily avoided by the overlord by keeping his monsters out of sight. They just move in to attack and after that disappear around a corner. That should be the standard tactic anyway to prevent that the heroes battle each turn...