The term you're looking for is "physically diverse". If you get both access circuits from the same provider, you should be able to achieve this, though it may cost. If you go with two different providers, you run into problems.
All the folks who own the fiber, AT&T, Sprint, MCI, WorldCom, etc. swap capacity as needed. If you buy from the same provider, all the circuit routing information will be in the same database. If you buy from two different providers, there will be no way to tell whether or not their two OC3's ride the same OC48 bundle.
Try to look at the Worldcom/fiber-cut threads of the last few days. There's some good info in them.