I, too, struggled with this issue (SD library documentation not up to date with latest release of Arduino 1.00).
After much trial and error, I found that, in the beginning of one's program, one simply needs to declare another file to exist. For example, I used:
Then just go about opening, closing, writing, reading, etc. each File in the normal way independent of the other.
I have tried this for just 2 files to be open at the same time. Worked great. I don't know how many FIles can be declared before Arduino complains.