Three-dimensional numerical computations have been carried out in flow past two inline finite-height circular cylinders using Open Source Field Operation and Manipulation. Investigations have been carried out for the varying Reynolds number (Re) ranging from 150 to 300. The diameter of the upstream cylinder is varied in such a way that the size ratio (SR, ratio of the diameter of the upstream cylinder to that of the downstream cylinder) takes values of 0.25, 0.5, and 1.0. For each size of the upstream cylinder, the downstream cylinder is placed at different locations in the streamwise direction. Effects of Re, SR, and inter-cylinder spacing (S) on three-dimensional unsteady wake characteristics behind upstream and downstream cylinders have been examined using iso-Q surfaces. Unsteady wake oscillations in both the wakes are analyzed qualitatively and quantitatively in terms of Hilbert spectra and the degree of stationarity using the transverse velocity component in the wake. Different flow regimes for upstream and downstream wakes have been identified and discussed with the change in Re, SR, and S. Transitions in the wake flow are illustrated using vorticity contours, frequency spectra, and bifurcation diagram. The level of wake synchronization in the upstream wake, downstream wake, and between both the wakes has been identified with the help of the cross correlation function. © 2021 Author(s).