  We study the relaxation process of two driven colloidal suspensions in
diffusive contact to a steady state, similar to thermalization. We start by
studying a single suspension, subjecting it to random driving forces via
holographic optical tweezers, which agitate it to a higher effective
temperature. Interestingly, the effective temperature of the suspension,
defined by the Einstein relation, exhibits a non-monotonic dependence on the
driving frequency. Next, we follow the flux of particles between two such
suspensions in diffusive contact, starting from a uniform density and relaxing
to a state with zero net particle flux. The density remains uniform for systems
with different frequencies but equal effective temperatures. At high driving
frequencies, we show that the density distribution at steady state is
determined by equating the ratio of the chemical potential to the effective
temperature in both systems, mirroring thermal equilibrium behavior.
