Are ambidextrous people better at multitasking?
This might not answer the question well enough to count as a good answer, but if you're interested in handedness in general, I highly recommend Stanley Coren's well-written and well-researched "The Left-Hander Syndrome."

If I recall correctly, the book doesn't address multitasking specifically, but it does address many issues associated with handedness, including lateralization of language and identity, development of mood disorders, and behavioral differences (especially risk of injury) between left- and right-handers. You might get some good pointers by reading the references.