  (abridged) Kepler-278 and Kepler-391 are two of the three evolved stars known
to date on the RGB to host multiple short-period transiting planets. Moreover,
these planets are among the smallest discovered around RGB stars. Here we
present a detailed stellar and planetary characterization of these remarkable
systems. Based on high-quality spectra from Gemini-GRACES for Kepler-278 and
Kepler-391, we obtained refined stellar parameters and precise chemical
abundances for 25 elements. Also, combining our new stellar parameters with a
photodynamical analysis of the Kepler light curves, we determined accurate
planetary properties of both systems. The precise spectroscopic parameters of
Kepler-278 and Kepler-391, along with their high
$^{12}\mathrm{C}/^{13}\mathrm{C}$ ratios, show that both stars are just
starting their ascent on the RGB. The planets Kepler-278b, Kepler-278c, and
Kepler-391c are warm sub-Neptunes, whilst Kepler-391b is a hot sub-Neptune that
falls in the hot super-Earth desert and, therefore, it might be undergoing
photoevaporation of its outer envelope. The high-precision obtained in the
transit times allowed us not only to confirm Kepler-278c's TTV signal, but also
to find evidence of a previously undetected TTV signal for the inner planet
Kepler-278b. From the presence of gravitational interaction between these
bodies we constrain, for the first time, the mass of Kepler-278b
($M_{\mathrm{p}}$ = 56 $\substack{+37\\-13}$ $M_{\mathrm{\oplus}}$) and
Kepler-278c ($M_{\mathrm{p}}$ = 35 $\substack{+9.9\\ -21} $
$M_{\mathrm{\oplus}}$). Finally, our photodynamical analysis also shows that
the orbits of both planets around Kepler-278 are highly eccentric ($e \sim$
0.7) and, surprisingly, coplanar. Further observations of this system are
needed to confirm the eccentricity values presented here.
