Split Fermentation, One Working, One Not

I just brewed up a 10.5 gallon ale this past weekend and put 5.25 gallons each into 2 primary bucket fermenters and added a vial of White Labs WLP 051 to each after aerating both with oxygen. Everything was the same, stored in the same room at the same temperature. Now one of them is working like crazy with the air lock bubbling away. The other ... nothing. I took the lid off to check it and it doesn't seem to be doing anything. Maybe I got a bad vial of yeast? One vial has a best used date of 12/15/12 while the other is 1/5/13. I'm not sure which one I put into which bucket though. Any ideas? Thanks.

it is probably just a difference in the viability of the yeast between the two vials, one obviously had a lower viability and is experiencing a longer lag time while the little guys build up their numbers. Don't worry about it, I'm sure before too long the other one will be chugging happily away as well.