You can just add another addFilter line manually like you have referenced... but I think your column name is wrong. I think it should be something like:
<?php
if (isset($_POST["doneRegister"]) || isset($_POST["doneRegister_x"])) {
$UpdateQuery = new WA_MySQLi_Query($bucketListHub);
$UpdateQuery->Action = "update";
$UpdateQuery->Table = "bucMylist";
$UpdateQuery->bindColumn("myl_done", "i", "".((isset($_POST["bucketDone"]))?$_POST["bucketDone"]:"") ."", "WA_DEFAULT");
$UpdateQuery->bindColumn("myl_doneDate", "t", "".((isset($_POST["bucketDoneDate"]))?$_POST["bucketDoneDate"]:"") ."", "WA_DEFAULT");
$UpdateQuery->bindColumn("myl_dateUpdate", "t", "", "WA_CURRENT_TIMESTAMP");
$UpdateQuery->addFilter("myl_bucID", "=", "d", "".((isset($_POST["buc_id"]))?$_POST["buc_id"]:"") ."");
$UpdateQuery->addFilter("myl_UserID", "=", "d", "".($_SESSION['SecurityAssist_UserID']) ."");
$UpdateQuery->execute();
$UpdateGoTo = "";
if (function_exists("rel2abs")) $UpdateGoTo = $UpdateGoTo?rel2abs($UpdateGoTo,dirname(__FILE__)):"";
$UpdateQuery->redirect($UpdateGoTo);
}
?>
I don't know the correct column name, so I used myl_UserID in my example... but I'm pretty sure the two filters wouldn't be set to the same column since it couldn't have two values simultaneously.