When Dokluwiki loads it looks for $conf['template'], which holds the name of your template. It's possible in doku.php to check the namespace and set $conf['template'] to the required template. You should be able to use getNS() to get the namespace or $INFO['namespace']. You have to do this after the initialization scripts have been loaded.
The code could look like this:
if($INFO['namespace'] == 'choice_1') {
$conf['namespace'] = 'namespace_1';
}
else $conf['namespace'] = 'namespace_2';