Can we try like:

function onetest1() {  

var nonmatchingVal = [], encodedquerystr = '';

var table1= new GlideRecord('u_table1');

table1.addNotNullQuery('field_name');

table1.query();

while(table1.next()){

var table2 = new GlideRecord('u_table2');  

if(table2.get('field_name', table1.field_name))

encodedquerystr = encodedquerystr + 'field_name!=' + table1.field_name + '^OR';

}

encodedquerystr = encodedquerystr.substring(0, encodedquerystr.length - 3).trim();

var table = new GlideRecord('u_table2');

table.addEncodedQuery(encodedquerystr);

table.query();

while(table.next())

nonmatchingVal.push(table.field_name.toString());

gs.print(nonmatchingVal);

}