The trustee should have issued a 1099-Q for each account. If used for education, the 1099-Q should be issued to the beneficiary, not the account owner.
You need to know the earnings and contribution for each account and that might not be half each. You'll need to get this information from the financial institution.
To avoid this problem in the future, have the beneficiaries make the distributions, or send the funds directly to the school. That way the 1099-Q will be issued in the student's name.
CLICK
HERE for more information on the 1099-Q
CLICK
HERE for a guide to the 1099-Q