Couldn't you add an action for Detect_Initial_Press_Only for the Shift key that increases your speed and then On_Key_Release decrese the speed back to a walking speed? OR
While Shift.isPressed() speed = RUN_SPEED otherwise speed = WALK_SPEED
I would try one of those as a work around, I have never actually tried to detect multiple keys being pressed at once and have them both firing off actions. I'm sure it's possible though, fighting games seem to be all about multi-button mashing. Let us know if you can't get anything working and I'll try and play with some code to experiment.