  This work provides a rigorous methodology for assessing the feasibility of
spectrum sharing between large low-earth orbit (LEO) satellite constellations.
For concreteness, we focus on the existing Starlink system and the
soon-to-be-launched Kuiper system, which is prohibited from inflicting
excessive interference onto the incumbent Starlink ground users. We carefully
model and study the potential downlink interference between the two systems and
investigate how strategic satellite selection may be used by Kuiper to serve
its ground users while also protecting Starlink ground users. We then extend
this notion of satellite selection to the case where Kuiper has limited
knowledge of Starlink's serving satellite. Our findings reveal that there is
always the potential for very high and extremely low interference, depending on
which Starlink and Kuiper satellites are being used to serve their users.
Consequently, we show that Kuiper can protect Starlink ground users with high
probability, by strategically selecting which of its satellites are used to
serve its ground users. Simultaneously, Kuiper is capable of delivering
near-maximal downlink SINR to its own ground users. This highlights a feasible
route to the coexistence of two dense LEO satellite systems, even in scenarios
where one system has limited knowledge of the other's serving satellites.
