Igen, már megoldottam. A végleges fv. így néz ki:

function changePartner1(partner1, id) {

var $selected = $(partner1).val();
var $url = $('input#searchAjaxPartner2').val();
var $span2 = 'partner2'+ id +'';

$.post($url, {partner1: $selected, id: id},
function(data){
if(data.length > 0) {
$('span#'+ $span2 +'').html(data);
}
});
}