you can call the script include on your 2nd field as advanced reference qualifier. Reference Qualifiers - ServiceNow Wiki

sample code, please modify per your need.

var GetListValue = Class.create();

GetListValue.prototype = {

  initialize: function() {

  },

  getList : function()

  {

  var ids = [];

  var gr = new GlideRecord('Your table name');

  var sys_id = this.getParameter();

  gr.addQuery('<sys_id variable>', sys_id);

  gr.query();

  while(gr.next())

  {

  ids.push(gr.sys_id.toString());

  }

  return 'sys_idIN' + ids;

  },

  type: 'GetListValue'

};

also, i think you can create a datal ookup between between these two fields.