The brain share still lives: Interestingly, I also just finished a book that switched back and forth too. One character was "historical" and his sections were told in the first person past tense. The present tense contemporary person's story was told in present tense third person. It took some getting used to, but in the end it was really deftly done--and really fit nicely with how the book resolved.

(If you tell me that you're also talking about a YA book by Susan Cooper called Victory I may officially become frightened.)