  We report the detection of X-ray pulsations from the rotation-powered
millisecond-period pulsars PSR J0740+6620 and PSR J1614-2230, two of the most
massive neutron stars known, using observations with the Neutron Star Interior
Composition Explorer (NICER). We also analyze X-ray Multi-Mirror Mission
(XMM-Newton) data for both pulsars to obtain their time-averaged fluxes and
study their respective X-ray fields. PSR J0740+6620 exhibits a broad
double-peaked profile with a separation of ~0.4 in phase. PSR J1614-2230, on
the other hand, has a broad single-peak profile. The broad modulations with
soft X-ray spectra of both pulsars are indicative of thermal radiation from one
or more small regions of the stellar surface. We show the NICER detections of
X-ray pulsations for both pulsars and also discuss the phase relationship to
their radio pulsations. In the case of PSR J0740+6620, this paper documents the
data reduction performed to obtain the pulsation detection and prepare for
pulse profile modeling analysis.
