  Derivation of physical properties of galaxies using spectral energy
distribution (SED) fitting is a powerful method, but can suffer from various
systematics arising from model assumptions. Previously, such biases were mostly
studied in the context of individual galaxies. In this study, we investigate
potential biases arising from performing the SED fitting on the combined light
of two galaxies, as would be the case in post-merger systems. We use
GALEX-SDSS-WISE Legacy Catalog (GSWLC) of z<0.3 galaxies to identify 9,000
galaxy pairs that could eventually merge. For these we investigate if the
UV/optical SED fitting accurately determines the stellar mass and (specific)
star formation rate if the pair was unresolved (merged). The sum of the stellar
masses (and SFRs) of individual galaxies in the pair establishes the ground
truth for these quantities. For star forming galaxies no biases (<0.1 dex) are
found in the stellar mass, SFR, or sSFRs. Moderate systematics in SFR (~0.1
dex) are found for systems with an extreme contrast in dust content between the
two galaxies. We conclude that biases that would arise in the determination of
masses and SFRs of post-merger systems on account of the two original galaxies
having potentially very different star formation histories and different dust
properties are small and that the approach with simple two-component star
formation histories is adequate. The approach presented in this study, using
flux compositing with empirically determined ground truth, offers new
opportunities for testing the results of SED fitting in general.
