can we perform clientside validation as well as serverside validation simultaneously

Yes you can. There is support for that in CActiveForm:

$form = $this->beginWidget('CActiveForm', array(






but you can use whatever clientside validation framework you want. server side just needs to receive some posted data and what happens with this data prior to send - does not matter.

Using clientside validation (not ajax validation) can reduce server usage as there is less invalid requests.