What undesired results? There's no such bug against the font packages,
and fonts-liberation2's README.Debian explicitly says installing both is
supposed to be OK.
Since having both fonts-liberation and fonts-liberation2 on
the system may lead to undesired results
flightgear uses these fonts via a symlink
usr/share/fonts/truetype/liberation. Hence, changing the dependency to
fonts-liberation2 would be easy (would that be an improvement?), but
making it ORed is harder. Maintscript checking which of the two exists
and linking to that one? and maybe a dpkg file trigger
(http://sources.debian.net/src/dpkg/1.18.18/doc/triggers.txt/) to catch
the case of replacing fonts-liberation by fonts-liberation2 when
flightgear is already installed?
I vaguely remember (from several upstream releases ago) that flightgear
works (but looks wrong) without these fonts, which would allow reducing
the dependency to a Recommends, but that would still install the
recommended font by default even if the other was already installed.