Its's works but a have a little inconvenient : 
pbs1       ns1           backup : A, B , C, D
synchro every day on pbs2 with a bigger retention.  -->   pbs  ns1 backup :  A,, B, C, D, E, F, G .... 
i wanted tu push back the backup  ( G for exemple )  to PBS 1 , in order to have  pbs1  ns1...