Try to retrieve the value you want from dropDown2, and set its disabled property to true, directly (there is no setter, it's just a plain javascript object). Then call dropDown2.startup() to make the changes on the UI. Example :
require(["dojo/_base/array"],
function(array) {
var self = this,
opt = array.filter(
self.dropDown2.options,
"return item.value == '" + self.dropDown2.get("value") + "'"
).pop();
opt.disabled = true;
this.startup();
});