waitasecond In a situation where a charge is from a diagonal and you take directly towards as meaning moving in a straight line to the center of the creature there are angles at which you cannot charge.
############ST
############C#
A#############
where A is the Attacker, T is the target, C is the closest square, and S is the square you go into on a straight line. In this situation the S is 13 squares and the C is 12. So obviously you have to go to C as it is the closest square. However if you rule that you have to follow a straight line, not only do you break the closest square rule, but you can't even get to S without a higher speed.
Directly toward therefore cannot equal to "following a straight line".
In the miniatures rules, the shortest distance is not a straight line.
If directly is taken to mean without deviation I would propose that it means without adding to the distance.
So yes by the rules as I interpret them two adventurers can charge at the same opponent without a problem. If you can't abstract the direct movement into squares, then drop the squares, and have the players move without 5 foot areas, and just the actually space a character takes up. Of course doing this will require a complete rewrite of the combat system. Seems better to simply abstract their movement out and understand these charging characters are not turning to hit at the last second. They are indeed moving "directly toward" their target.