  We propose a method to calculate the two-dimensional (2D) fission-fragment
yield $Y(Z,N)$ versus both proton and neutron number, with inclusion of
odd-even staggering effects in both variables. The approach is to use Brownian
shape-motion on a macroscopic-microscopic potential-energy surface which, for a
particular compound system is calculated versus four shape variables:
elongation (quadrupole moment $Q_2$), neck $d$, left nascent fragment
spheroidal deformation $\epsilon_{\rm f1}$, right nascent fragment deformation
$\epsilon_{\rm f2}$ and two asymmetry variables, namely proton and neutron
numbers in each of the two fragments. The extension of previous models 1)
introduces a method to calculate this generalized potential-energy function and
2) allows the correlated transfer of nucleon pairs in one step, in addition to
sequential transfer. In the previous version the potential energy was
calculated as a function of $Z$ and $N$ of the compound system and its shape,
including the asymmetry of the shape. We outline here how to generalize the
model from the "compound-system" model to a model where the emerging fragment
proton and neutron numbers also enter, over and above the compound system
composition.
