  As one of the main formation mechanisms of solar filament formation, the
chromospheric evaporation-coronal condensation model has been confirmed by
numerical simulations to explain the formation of filament threads very well in
flux tubes with single dips. However, coronal magnetic extrapolations indicated
that some magnetic field lines might possess more than one dip. It is expected
that the formation process would be significantly different in this case
compared to a single-dipped magnetic flux tube. In this paper, based on the
evaporation-condensation model, we study filament thread formation in
double-dipped magnetic flux tubes by numerical simulations. We find that only
with particular combinations of magnetic configuration and heating, e.g.,
concentrated localized heating and a long magnetic flux tube with deep dips,
can two threads form and persist in a double-dipped magnetic flux tube.
Comparing our parametric survey with observations, we conclude that such
magnetically connected threads due to multiple dips are more likely to exist in
quiescent filaments than in active-region filaments. Moreover, we find that
these threads are usually shorter than independently trapped threads, which
might be one of the reasons why quiescent filaments have short threads. These
characteristics of magnetically connected threads could also explain barbs and
vertical threads in quiescent filaments.
