Depends on their velocity relative to Earth. The unintuitive bit of relativity is that you observe the other as experiencing time more slowly - this is the orig

Depends on their velocity relative to Earth. The unintuitive bit of relativity is that you observe the other as experiencing time more slowly - this is the origin of the twin paradox (i.e., shouldn't each twin observe the other as having aged less?) whose resolution comes from the fact that the second twin is in a non-inertial reference frame once he changes directions to return to Earth.