  In the above letter, Chin and Chen proposed an IEEE 1588 clock
synchronization method based on dual slave clocks, where they claim that
multiple unknown parameters --- i.e., clock offset, clock skew, and
master-to-slave delay --- can be estimated with only one-way time transfers
using more equations than usual. This comment investigates Chin and Chen's dual
clock scheme with detailed models for a master and dual slave clocks and shows
that the formulation of multi-parameter estimation is invalid, which affirms
that it is impossible to distinguish the effect of delay from that of clock
offset at a slave even with dual slave clocks.
