Effettivamente prima non si capiva

<input type="button" value="Go"
onClick="location=document.f1.s1.options[document.f1.s1.selectedIndex].value + '?s2=' + document.f1.s2.options[document.f1.s2.selectedIndex].value">

ma puoi semplificare almeno un pochino:

<input type="button" value="Go"
onClick="mf=document.f1;location=mf.s1.options[mf.s1.selectedIndex].value + '?s2=' + mf.s2.options[mf.s2.selectedIndex].value">

Io comunque riscriverei tutto

<form name="f1" action="index.php">
<select name="order">
<option value="1">#</option>
<option value="2">#</option>
<option value="3">#</option>
</select>

<select name="s2" class="myselect">
<option value="br2">Crescente</option>
<option value="br1">Descresnete</option>
</select>

<input type="button" value="Go"
onClick="this.form.submit()">
</form>

ciao