  We report on long-term X-ray timing of two anomalous X-ray pulsars, 1RXS
J170849.0-400910 and 1E 2259+586, using the Rossi X-ray Timing Explorer. In
monthly observations made over 1.4 yr and 2.6 yr for the two pulsars,
respectively, we have obtained phase-coherent timing solutions which imply that
these objects have been rotating with great stability throughout the course of
our observations. For 1RXS J170849.0-400910, we find a rotation frequency of
0.0909169331(5) Hz and frequency derivative -15.687(4) x 10^(-14) Hz/s, for
epoch MJD 51215.931. For 1E 2259+586, we find a rotation frequency of
0.1432880613(2)Hz, and frequency derivative -1.0026(7) x 10^(-14) Hz/s, for
epoch MJD 51195.583. RMS phase residuals from these simple models are only
about 0.01 cycles for both sources. We show that the frequency derivative for
1E 2259+586 is inconsistent with that inferred from incoherent frequency
observations made over the last 20 yr. Our observations are consistent with the
magnetar hypothesis and make binary accretion scenarios appear unlikely.
