Counting backtracks with several calls to fd_labeling

Dear All,

I can't figure out a way of properly counting the (total) number of 
backtracks with the finite domain solver in presence of several calls to 
fd_labeling, like in the following example (where my naïve attempt 
obviously fails):

solve(Xs, Ys, B):-
     [... define model with variables Xs and Ys ...]

     fd_labeling(Xs, [backtracks(BX)]),
     fd_labeling(Ys, [backtracks(BY)]),
     B is BX+BY,

     [... print solution ...].

Any suggestion?

Best regards,

-- Nicolas