  I find that Type Ia supernovae (SNe Ia) with bimodal nebular emission
profiles occur almost exclusively in massive ($M_\star \gtrsim
10^{11}~M_\odot$) galaxies with low star-formation rates (SFR~$\lesssim
0.5~M_\odot$/yr). The bimodal profiles are likely produced by two white dwarfs
that exploded during a merger or collision, supported by a correlation between
the peak-to-peak velocity separation ($v_{\rm sep}$) and the SN Ia peak
luminosity ($M_V$) which arises naturally from more massive white dwarf
binaries synthesizing more $^{56}$Ni during the explosion. The quiescent hosts
are consistent with the long delay times required to form double white dwarf
binaries. The distributions of SNe Ia with and without bimodal nebular lines
differ in host mass, SFR, and specific SFR with K-S test probabilities of
$3.1\%$, $0.03\%$, and $0.02\%$, respectively. Viewing angle effects can fully
explain the SNe Ia in quiescent hosts without bimodal emission profiles and the
dearth of merger/collision driven SNe Ia in star-forming hosts requires at
least two distinct progenitor channels for normal SNe Ia. $30-40\%$ of all SNe
Ia originate from mergers or collisions depending on how cleanly host
environment distinguishes progenitor scenarios. The bimodal SNe Ia share some
characteristics with the underluminous 91bg-like SNe Ia that also prefer older
populations, but there is no unambiguous connection between the two
classifications. This may suggest separate processes or multiple axes of ejecta
(a)symmetry. Existing models for WD mergers and collisions broadly reproduce
the $v_{\rm sep} - M_V$ correlation and future analyses may be able to infer
the masses/mass-ratios of merging white dwarfs in external galaxies.
