  A leading model for Type Ia supernovae involves the double-detonation of a
sub-Chandrasekhar mass white dwarf. Double-detonations arise when a surface
helium shell detonation generates shockwaves that trigger a core detonation;
this mechanism may be triggered via accretion or during the merger of binaries.
Most previous double-detonation simulations only included the primary white
dwarf; however, the fate of the secondary has significant observational
consequences. Recently, hydrodynamic simulations accounted for the companion in
double-degenerate double-detonation mergers. In the merger of a
1.05$\text{M}_{\odot}$ primary white dwarf and 0.7$\text{M}_{\odot}$ secondary
white dwarf, the primary consistently detonates while the fate of the secondary
remains uncertain. We consider two versions of this scenario, one in which the
secondary survives and another in which it detonates. We present the first 3D
radiative transfer calculations for these models and show that the synthetic
observables for both models are similar and match properties of the peculiar
02es-like subclass of Type Ia supernovae. Our calculations show angle
dependencies sensitive to the companion's fate, and we can obtain a closer
spectroscopic match to normal Type Ia supernovae when the secondary detonates
and the effects of helium detonation ash are minimised. The asymmetry in the
width-luminosity relationship is comparable to previous double-detonation
models, but the overall spread is increased with a secondary detonation. The
secondary detonation has a meaningful impact on all synthetic observables;
however, multidimensional nebular phase calculations are needed to support or
rule out either model as a likely explanation for Type Ia supernovae.
