  We propose a method for simulating a single realization of a collision of two
Bose--Einstein condensates. Recently in Zi\'{n} {\it et al.} (Phys. Rev. Lett.
{\bf 94}, 200401 (2005)) we introduced a quantum model of an incoherent elastic
scattering in a collision of two counter-propagating atomic Gaussian
wavepackets. Here show that this model is capable of generating the data that
can be interpreted as results of a single collision event. We find a range of
parameters, including relative velocity, population and the size of colliding
condensates, where the structure of hallo of scattered atoms in a single
realization strongly differs from that averaged over many realizations.
