When was it earned?   If was earned evenly during the year, split it based on the number of days being reported for each return.  As a side note, you won't have any luck getting a new 1099 issued.  To make myself feel better, I have always attached a note to both returns showing how the interest was split between years.  I don't know if was ever read by anyone at the IRS, but I never had to respond to any notices either so it at least worked as a good luck charm.