Try
serialize()
both of them together.
var bookingdate = $(this).parent().parent().find("#bookingdate"),
merchant_id = $(this).parent().parent().find("#merchant_id"),
filterdata = bookingdate.add(merchant_id).serialize();
$.post("<?php echo Router::url(array('controller'=>'MCP','action'=>'getMtRestaurant')); ?>",
filterdata, function () {})