Well, it's difficult to have more than one last name...
You could use Christina Baker-Kline or Christina Baker_Kline.
You could perhaps also use a non-breaking space between the surnames. I haven't tested, not near my computer. It would look like a space but the two surnames will (perhaps) be treated like one big surname as when a hyphen is used.
http://en.m.wikipedia.org/wiki/Non-breaking_space