<select name="timedo">
        <option value="08">08:00</option>
        <option value="09">09:00</option>
</select>

<iframe src="mydoc.php?id=[B][timedo][/B]" id="frame" width="400" height="400"></iframe> 

<script>
document.querySelector("select[name='timedo']").onchange = function(){document.getElementById("frame").src = "mydoc.php?id="+this.value }
</script>

<select name="timedo">
        <option value="08">08:00</option>
        <option value="09">09:00</option>
</select>
<select name="datedo">
        <option value="20">20.01</option>
        <option value="21">21.01</option>
</select>

<iframe src="mydoc.php?id=[B][timedo][/B]" id="frame" width="400" height="400"></iframe> 

<script>

for(var fr = document.getElementById("frame"), t = document.getElementsByTagName("select"),i=0,b=t.length;i<b;i++){

if ( t[i].name !== "timedo" || t[i].name !== "datedo") continue;

t[i].onchange = function(){

fr.src = "mydoc.php?timedo="+t[0].value+"&datedo="+t[1].value;

}


}

</script>