  Earth-scale planets in the classical habitable zone (HZ) are more likely to
be habitable if they possess active geophysics. Without a constant internal
energy source, planets cool as they age, eventually terminating tectonic
activity and rendering the planet sterile to life. However, for planets
orbiting low-mass stars, the presence of an outer companion could generate
enough tidal heat in the HZ planet to prevent such cooling. The range of mass
and orbital parameters for the companion that give adequate long-term heating
of the inner HZ planet, while avoiding very early total desiccation, is
probably substantial. We locate the ideal location for the outer of a pair of
planets, under the assumption that the inner planet has the same incident flux
as Earth, orbiting example stars: a generic late M dwarf ($T_{eff}=2670 K$) and
the M9V/L0 dwarf DEN1048. Thus discoveries of Earth-scale planets in the HZ
zone of old small stars should be followed by searches for outer companion
planets that might be essential for current habitability.
