We present a study of hopping conductivity for a system of sites that
can be occupied by more than one electron. At a moderate on-site Coulo
mb repulsion, the coexistence of sites with occupation numbers 0, 1, a
nd 2 results in an exponential dependence of the Mott conductivity upo
n Zeeman splitting H-mu B. We show that the conductivity behaves as 1n
sigma = (T/T-0)F-1/4(x), where F is a universal scaling function of x
= H-mu B/T(T-0/T)(1/4). We find F(x) analytically at weak fields, x m
uch less than 1, using a perturbative approach. Above some threshold x
(th), the function F(x) attains a constant value, which is also found
analytically The full shape of the scaling function is determined nume
rically, from a simulation of the corresponding ''two-color'' dimensio
nless percolation problem. In addition, we develop an approximate meth
od which enables us to solve this percolation problem analytically at
any magnetic field. This method gives a satisfactory extrapolation of
the function F(x) between its two limiting forms.