Is this if-else-condition possible in layout XML?

There is no "inverse directive" to ifconfig. You can still have the name parameter calculated by a helper method like this:

XML:

<action method="addItem">
    <type>js_css</type>
    <name helper="helpername/helpermethod"/>
</action>

helpername is the identifier that you usually pass to Mage::helper($name) and helpermethod the method you want to call.

In your module's helper class (pseudo code):

public function helpermethod()
{
    if ($enabled) {
        return 'module/app1.css';
    } else {
        return 'module/app2.css';
    }
}

use this extension for extended config option

you can use condition tag for tag

<action method="addItem" ifconfig="module/general/enable" condition="1">
    <type>js_css</type>
    <name>module/app.css</name>
</action>
<action method="addItem" ifconfig="module/general/enable" condition="0">
    <type>js_css</type>
    <name>module/app1.css</name>
</action>