  Free-floating planets are the remnants of violent dynamical rearrangements of
planetary systems. It is possible that even our own solar system ejected a
large planet early in its evolution. WFIRST will have the ability to detect
free-floating planets over a wide range of masses, but it will not be able to
directly measure their masses. Microlensing parallax observations can be used
to measure the masses of isolated objects, including free-floating planets, by
observing their microlensing events from two locations. The intra-L2 separation
between WFIRST and Euclid is large enough to enable microlensing parallax
measurements, especially given the exquisite photometric precision that both
spacecraft are capable of over wide fields. In this white paper we describe how
a modest investment of observing time could yield hundreds of parallax
measurements for WFIRST's bound and free-floating planets. We also describe how
a short observing campaign of precursor observations by Euclid can improve
WFIRST's bound planet and host star mass measurements.
