ผมต้องการ Disable 2 ฟิล์ด ครับ เขียนแบบข้างล่างมันไม่ทำงานให้ครับ

<ul>
	<li><input name="txt_other" type="radio" value="not_study" id="txt_other1" onClick="if(this.checked==true || this.checked=='checked'){document.getElementById('txt_other_name').disabled=true;
	document.getElementById('txt_other_field').disabled=true;}"/>&nbsp;not present study</li>
    </ul>
    <ul>
	<li><input name="txt_other" type="radio" id="txt_other2" value="study" onClick="if(this.checked==true || this.checked=='checked'){document.getElementById('txt_other_name').disabled=false;
document.getElementById('txt_other_field').disabled=false;}"/>&nbsp;study towards</li>
        <li>name of institution : <input name="txt_other_name" type="text" /></li>
        <li>field : <input name="txt_other_field" type="text" /></li>
    </ul>

if(document.application_from.txt_healths1.checked == false && document.application_from.txt_healths2.checked == false )
	{
		alert('Please select answer of health title 1');
		return false;
	}
	/*if(document.application_from.txt_yes_con.value == "")
	{
		alert('Please enter a question about your health title 1');
		document.application_from.txt_yes_con.focus();		
		return false;
	}*/