Device-device skew is measured by inputting the same clock into two devices (CY230x) at the same time and measuring the largest difference in time between any two outputs (from both devices). This is essentially a parallel connection/measurement. This is done for several parts over temperature, voltage, and frequency. The largest difference between any two outputs, plus margin, is 700 ps spec. Please refer to the datasheet for the values specified.
If you connect the devices in series, there is a spec t6A (delay, REF to CLKOUT) that also needs to be added. Thus, the output to output skew grows when connected in series. This is essentially the propagation delay in passing through the device.