You have to put a comma betwen the two columns, try this code:
$upd_question = "UPDATE `question_bank` qtbl INNER JOIN `answer_bank` atbl
SET qtbl.`question`='".$edit_question."',
atbl.`answer`= CASE WHEN atbl.`aid`='".$aid[0]."' THEN '".$edit_ans1."'
WHEN atbl.`aid`='".$aid[1]."' THEN '".$edit_ans2."'
WHEN atbl.`aid`='".$aid[2]."' THEN '".$edit_ans3."'
END
,atbl.`correct` = CASE WHEN ".$edit_correct."=='1' THEN '1'
WHEN ".$edit_correct."=='2' THEN '1'
WHEN ".$edit_correct."=='3' THEN '1'
ELSE '0'
END
WHERE qtbl.`qid`=atbl.`question_id` AND qtbl.`qid`='".$edit_qid."'";
mysqli_query($mysqli, $upd_question) or die ("<b>Update of question failed:</b> " . mysqli_error($mysqli));
At the second case you can use IF() sentence, using IF will be more readable code.