The flatness of chaos generated by mutually coupled vertical-cavity surface-emitting lasers (VCSELs) has been investigated experimentally in two configurations. Power variation between dc and the bandwidth frequency and flat bandwidth are used to quantitatively study the flatness of chaos. The power spectrum of chaos from a single VCSEL in both configurations is not flat due to a peak at relaxation oscillation frequency. This uneven nature will affect the application of chaos in random number generation. The experimental results show that the power spectra of chaos can easily be flattened by combining two uneven chaos. Flat bandwidth of the combined output can be 15 times higher than that of chaos from a single VCSEL at the optimum frequency detuning. The mechanism for this flattening is attributed to the frequency beating.