Time dilation due to velocity has been confirmed from Earth's frame of reference since the Rossi and Hall (1940) muon measurements, but I am not aware of tests made from two frames in relative uniform motion. Of course, no one made measurements of Earth's clocks from the reference frame of the muons showering Earth at very close to c to see if Earth was time dilated from that frame.

Our propulsion systems can only get our clocks moving at negligible fractions of c; however, our atomic clocks are (I think) sufficiently fast and precise. As I understand it, we could fly two atomic clocks past each other while they exchange signals, measuring each other's time dilation. If each measures the other as slower, STR is further confirmed.

If the answer to my above question is No, can anyone weigh in on whether my proposed test is feasible?