  As the Kepler mission has done for hot exoplanets, the ESA Euclid and NASA
Roman missions have the potential to create a breakthrough in our understanding
of the demographics of cool exoplanets, including unbound, or "free-floating",
planets (FFPs). In this study, we demonstrate the complementarity of the two
missions and propose two joint-surveys to better constrain the mass and
distance of microlensing events. We first demonstrate that an early brief
Euclid survey (7 h) of the Roman microlensing fields will allow the measurement
of a large fraction of events relative proper motions and lens magnitudes.
Then, we study the potential of simultaneous observations by Roman and Euclid
to enable the measurement of the microlensing parallax for the shortest
microlensing events. Using detailed simulations of the joint detection yield we
show that within one year Roman-Euclid observations will be at least an order
of magnitude more sensitive than current ground-based measurements. Depending
on the exact distribution of FFP, a joint Roman-Euclid campaign should detect
around 130 FFP events within a year, including 110 with measured parallax that
strongly constrain the FFP mass, and around 30 FFP events with direct mass and
distance measurements. The ability of the joint survey to completely break the
microlens mass-distance-velocity degeneracy for a significant subset of events
provides a unique opportunity to verify unambiguously the FFP hypothesis or
else place abundance limits for FFPs between Earth and Jupiter masses that are
up to two orders of magnitude stronger than provided by ground-based surveys.
Finally, we study the capabilities of the joint survey to enhance the detection
and charcterization of exomoons, and found that it could lead to the detection
of the first exomoon.
