  We present a model to study secularly and tidally evolving three-body systems
composed by two low-mass planets orbiting a star, in the case where the bodies
rotation axes are always perpendicular to the orbital plane. The tidal theory
allows us to study the spin and orbit evolution of both stiff Earth-like
planets and predominantly gaseous Neptune-like planets. The model is applied to
study two recently-discovered exoplanetary systems containing potentially
habitable exoplanets (PHE): LHS-1140 b-c and K2-18 b-c. For the former system,
we show that both LHS-1140 b and c must be in nearly-circular orbits. For K2-18
b-c, the combined analysis of orbital evolution timescales with the current
eccentricity estimation of K2-18 b allows us to conclude that the inner planet
(K2-18 c) must be a Neptune-like gaseous body. Only this would allow for the
eccentricity of K2-18 b to be in the range of values estimated in recent works
($e=0.20 \pm 0.08$), provided that the uniform viscosity coefficient of K2-18 b
is greater than $2.4 \times 10^{19} \ \textrm{Pa s}$ (which is a value
characteristic of stiff bodies) and supposing that such system has an age of
some Gyr.
