You are correct that the twins can potentially perform fly-bys of synchronized, stationary clocks in order to compare their clocks again.
In the context of the twin paradox, where one twin stays on Earth while the other travels through space at relativistic speeds and then returns, it is indeed possible for them to perform additional clock comparisons during their journeys.
As the traveling twin moves away from Earth, they can encounter synchronized clocks that are stationary with respect to Earth. By comparing their onboard clock with these stationary clocks during a fly-by, they can assess the time dilation experienced due to their relative motion.
Similarly, on their return journey, the traveling twin can again perform fly-bys of synchronized, stationary clocks to compare their onboard clock's time with the clocks on Earth. These additional clock comparisons help demonstrate the effects of time dilation and allow for the verification of the twin paradox.
Therefore, the twins can certainly conduct fly-bys of synchronized clocks to perform subsequent clock comparisons, enabling them to observe and measure the effects of time dilation resulting from their relative motion.