I did get a reply and maybe somebody will be interested: The answer came from a person with the name 'Masamune' Thanks a lot!
Code:
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<mx:Script>
<![CDATA[
protected function name_changeHandler(event:Event):void {
var firstName:String = fName.text.toLowerCase();
var lastName:String = lName.text.toLowerCase();
fullName.text = firstName + lastName;
}
]]>
</mx:Script>
<mx:Label x="156" y="74" text="first"/>
<mx:TextInput x="199" y="72" id="fName" change="name_changeHandler(event)"/>
<mx:Label x="156" y="104" text="last"/>
<mx:TextInput x="199" y="102" id="lName" change="name_changeHandler(event)"/>
<mx:Text x="199" y="132" text="Text" id="fullName"/>
</mx:Application>