  We investigate within the Gross-Pitaevski (GP) theory the formation of fringe
patterns between two Bose condensates stored in a double well potential
modelling the forces applied to the experimental systems. In the case of
repulsive interactions between the atoms, we report the onset of interference
structures similar to those observed in the experiments, after the release of
the confining potential. Conversely, attractive interactions lead to the
collapse of the condensate when the number of particles is larger than a
critical value. We show that a bias field introduced in the center of the trap
allows the system to avoid the blow-up of the density and gives rise to a
periodic behavior of growth and decay of spatial modulations reminiscent of the
Fermi-Pasta-Ulam recurrence.
