Well, you can really do some cool things with two as it pertains to redundancy as you can link the two for redundant routing (VRRP), you don't have "all of your eggs in one basket", so even if you suffer a complete switch failure, you still have that other switch's connected users and devices still working, etc...
You could also look into the 8200 series as that chassis allows to populate with redundant management module if you wanted to stick with one chassis but have the highest level of hardware redundancy. Both the 5400 and 8200 series offers redundant power supplies, fans, and even suffering a port blade failure does not take the entire switch down. So the redundancy difference between those two series is the additional processor/management module that can be done on the 8200 series. Of course, this would use up a slot.
That being said, I'm a big fan of that 5400 series switch, they are rock solid, and if you have the money, I'd say go with the dual switches...but that's just my opinion.