  The existence of exchange-coupled Ni$^{2+}$ ions -- the so-called magnetic
dimers -- in wolframite-type NiWO$_4$ and Zn$_c$Ni$_{1-c}$WO$_4$ solid
solutions with high nickel content was discovered by X-ray absorption
spectroscopy combined with reverse Monte Carlo (RMC) simulations. Temperature-
(10--300 K) and composition-dependent x-ray absorption spectra were measured at
the Ni K-edge, Zn K-edge, and W L$_3$-edge of microcrystalline NiWO$_4$,
Zn$_c$Ni$_{1-c}$WO$_4$ and ZnWO$_4$. Structural models were obtained from
simultaneous analysis of the extended x-ray absorption fine structure (EXAFS)
spectra at three metal absorption edges using RMC simulations. The obtained
radial distribution functions for different atomic pairs made it possible to
trace in detail the changes in the local environment of metal ions and the
effect of thermal disorder. Dimerization of Ni$^{2+}$ ions within
quasi-one-dimensional zigzag chains of [NiO$_6$] octahedra was evidenced in
NiWO$_4$ in the whole studied temperature range. It manifests itself as the
splitting of the Ni-Ni radial distribution function into two separate peaks.
The effect is further preserved in solid solutions Zn$_c$Ni$_{1-c}$WO$_4$ for
$c \leq 0.6$, which is related to the probability to find two Ni$^{2+}$ ions in
neighboring positions.
