Automatically uncheck a checkbox when another checkbox is checked

I work with XUL and I have two hboxes in each there is one checkbox :

<hbox id="hBox1">
   <label value="label1" style="width:15ex" />
   <spacer style="width:5px" />
   <checkbox id="checkBox1" style="width: 2ex" />
</hbox>

<hbox id="hBox2">
   <label value="label2" style="width:15ex" />
   <spacer style="width:5px" />
   <checkbox id="checkBox2" style="width: 2ex" />
</hbox>

I want to have this behaviour : only one of the two chekboxes can be checked i.e. if the user checks one checkbox whereas the other is checked, then the second checkbox must be automatically unchecked

I tried this :

<checkbox id="checkBox1" style="width: 2ex" onchange="if (this.checked) document.getElementById('checkBox2').checked = false" />

<checkbox id="checkBox2" style="width: 2ex" onchange="if (this.checked) document.getElementById('checkBox1').checked = false" />

but it does not work