  Recent observational campaigns have shown that multi-planet systems seem to
be abundant in our Galaxy. Moreover, it seems that these systems might have
distant companions, either planets, brown-dwarfs or other stellar objects.
These companions might be inclined with respect to the inner planets, and could
potentially excite the eccentricities of the inner planets through the
Eccentric Kozai-Lidov mechanism. These eccentricity excitations could perhaps
cause the inner orbits to cross, disrupting the inner system. We study the
stability of two-planet systems in the presence of a distant, inclined, giant
planet. Specifically, we derive a stability criterion, which depends on the
companion's separation and eccentricity. We show that our analytic criterion
agrees with the results obtained from numerically integrating an ensemble of
systems. Finally, as a potential proof-of-concept, we provide a set of
predictions for the parameter space that allows the existence of planetary
companions for the Kepler-419, Kepler-56, Kepler-448, Kepler-88, Kepler-109,
and Kepler-36 systems.
