Solution found, i've edited the post and made a jsFiddle here: http://jsfiddle.net/santirisco/mPNsC/3/
$( ".slider-form").slider({
values: [0,0],
min: -5,
max: 5,
step:.5,
animate: true,
create: function(event, ui) {
//On Create create an array to store the values of each handler
hndlrpos=[];
},
stop: function( event, ui ) {
//On Stop store the values of each handler
for (i=0; i<=$('.ui-slider-handle').length; i++){
hndlrpos[i]=ui.values[i]
}
},
slide: function( event, ui ) {
//ON Slide return false if we are overpasing the limits,
//this part shuold be improved but by now it's working
if( (hndlrpos[0]!== undefined && hndlrpos[0]>0&&(ui.values[1])>0)
||
(hndlrpos[0]!== undefined && hndlrpos[0]<0&&(ui.values[1])<0)
||
(hndlrpos[1]!== undefined && hndlrpos[1]>0&&(ui.values[0])>0)
||
(hndlrpos[1]!== undefined && hndlrpos[1]<0&&(ui.values[0])<0))
{
return false;
}
}
});