  The iron lines at 630.15 and 630.25 nm are often used to determine the
physical conditions of the solar photosphere. A common approach is to invert
them simultaneously under the Milne-Eddington approximation. The same
thermodynamic parameters are employed for the two lines, except for their
opacities, which are assumed to have a constant ratio. We aim at investigating
the validity of this assumption, since the two lines are not exactly the same.
We use magnetohydrodynamic simulations of the quiet Sun to examine the behavior
of the ME thermodynamic parameters and their influence on the retrieval of
vector magnetic fields and flow velocities. Our analysis shows that the two
lines can be coupled and inverted simultaneously using the same thermodynamic
parameters and a constant opacity ratio. The inversion of two lines is
significantly more accurate than single-line inversions because of the larger
number of observables.
