Checkbox 1 is disabled if Checkbox 2 is checked

I have two checkboxes in Firefox addon's options. I need checkbox one be disabled if checkbox two is checked.

Function "onOpenedOptions" checks true/false (checked/unchecked) of variable (checkbox two) when options are opened & disables checkbox one if two is checked.

And I need to change the "disabled attribute" of checkbox one when I've already opened options & check/uncheck checkbox two. Here is "onChangedCheckbox" function, but if(document.getElementById('checkboxTwo').checked) doesn't work

How to verify if checkboxTwo is checked?

Also document.querySelector('#checkboxTwo') doesn't work. That's why I'm just using document now

options.xul:

<?xml version="1.0"?>
<?xml-stylesheet href="chrome://global/skin/" type="text/css"?>
<prefwindow id="test_prefwindow" title="Test" 
    xmlns="http://ift.tt/ZJ3Hg8">
    <prefpane id="test_prefpane" label="Preferences" >
        <preferences>
            <preference id="checkboxOne" name="extensions.checkboxOne" type="bool"/>
            <preference id="checkboxTwo" name="extensions.checkboxTwo" type="bool"/>
        </preferences>

        <row style="display:inline-block">
            <checkbox id="checkboxOne" preference="checkboxOne" label="Checkbox One" style="vertical-align:middle;"/>
            <checkbox id="checkboxTwo" preference="checkboxTwo" label="Checkbox Two" style="vertical-align:middle;"/>
        </row>
    </prefpane>
    <script type="application/x-javascript" src="chrome://test/content/options.js"/>
</prefwindow>

options.js:

function onOpenedOptions() {
    var prefManager = Components.classes["@http://ift.tt/1kVDlSr"].getService(Components.interfaces.nsIPrefBranch);
    if (prefManager.getBoolPref("extensions.checkboxTwo"))
        document.getElementById('checkboxOne').disabled=true;
}
document.addEventListener('DOMContentLoaded', onOpenedOptions);

function onChangedCheckbox() {
    if (document.getElementById('checkboxTwo').checked) 
        document.getElementById('checkboxOne').disabled=true;
    else document.getElementById('checkboxOne').disabled=false;
}
document.addEventListener("click", onChangeCheckbox);
//document.querySelector('#checkboxTwo').addEventListener("click", onChangedCheckbox);